Somewhere way back in your training, you were probably introduced to the Incident Pit. It’s a fairly dramatic diagram with the words “fatal” and “death” appearing at the bottom. The concept was meant to inspire you to deal with problems early, keep control and stay safe, with the tag line “Don’t fall in.” This always seemed to be a slightly strange thing to say to divers, who are destined to fall in (to a body of water) quite deliberately. Other than encouraging you to run through your own personal risk assessment for a dive, that’s about all the standard training material has to say about pre-dive thinking.
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In many sports the technique of visualisation is used to help elite athletes achieve their potential, and there’s good evidence that it works at lower levels too. Visualisation is the process of creating a mental image of what you want to happen or feel in reality. An athlete could use this technique to picture crossing the finishing line first. For a diver there is clear potential to visualise a relaxed, in control dive and achieve a state of calm and well-being before a dive. In fact, most people naturally tend to think through and rehearse what’s about to happen to them. So quite possibly you have started to think this way already.

Sometimes these preparatory thoughts can be plagued with recurring images of past mistakes or near misses, and that’s not conducive to ensuring the success of the upcoming dive. It can be more helpful to actively direct your pre-dive thoughts and control those images in your head. And the visualisation probably needs to be more than just a visual experience. To really be successful, you would need to focus on all the senses; the rush of cold water, the smell of the sea, the feeling of pressure on your legs as you enter the water, the sound of your bubbles.

When you visualise the successful dive, you are stimulating the same regions of your brain as you do when you physically perform the same action. So, thinking the dive through, with all it’s stages, is a way of conditioning your brain for a successful outcome. Perhaps the visualisation starts before the dive, right back to the preparing your equipment. And the beauty of this preparation is that you could be doing it anywhere. Picture being on the bus thinking about packing your kit for the weekend.

What if we extend this to thinking through the different situations that may arise underwater? Rather than the scary prospect of the Incident Pit, why not challenge divers to visualise their response to common problems? What would they do if their torch fails on a night dive? It’s too easy to just say “I’d get my back up torch out”. In order to visualise it you would have to work through being able to open your BCD pocket (feel the zipper in your hand, hear the zip and feel as it bumps along the teeth), feel through the gloves for the piston clip, unclip the torch lanyard from the D ring, feel the lanyard in your hand etc. Which pocket? What else in in there to avoid dislodging? How will the torch feel in my hand? This is a far more powerful psychological technique than a glib “Get my back up” response.

For divers just starting out on their diving journey, visualisation can be an excellent way to deal with the nervous trainee. Try to remember the first time you put all your dive kit on. How tight did your neck seal feel? How restricted was your movement? These are things we all take for granted now. We have done it often enough that we barely register the sensation. I think there is a real value in talking through that first dive. This is not part of your SEEDS brief, this is so much more. Find a quiet space, sit down and step your way through the dive, start building the neuronal connections in a positive way and help boost performance.

It doesn’t matter how carefully you monitor your fluid intake before a dive, or how close to dive time you leave the last toilet visit, we have all got out from a dive ready to rugby tackle anyone standing between us and the toilet. It’s well known that going for a dive causes an increased need to urinate. There are some interesting physiological changes that come to play. Immersion and temperature changes cause a narrowing of the blood vessels in the extremities. This results in an increased volume of blood to the central organs which is interpreted by the body as fluid overload. This causes the production of antidiuretic hormone (ADH) to stop, signalling to your kidneys that they need to produce urine to lower the blood volume. So even if your last toilet visit was only minutes ago, you can quickly find yourself feeling the need to go again.
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It’s almost impossible to give an exact measurement for the volume of the human bladder as everyone’s ability to hold urine varies. Normal adult bladders hold between 300 and 400ml but can hold up to 600 or even 1000ml in some cases. The need to urinate is stimulated by the expansion of the bladder which triggers the Micturition reflex centre in the spinal cord. Most adults will feel the need to urinate when their bladder is only around a quarter to a third of its normal capacity. In normal circumstances, adults will feel the need to empty their bladder about every 3 hours, but as divers we know the effect that immersion plays.

There’s always a big debate in the wetsuit diving community about whether peeing in a wetsuit is acceptable or not. In a drysuit, the debate becomes somewhat redundant unless you have a P-valve fitted. But is there any danger to ‘holding it in’? When you first feel the need to pee, your bladder probably has quite a way to go before it’s completely full. As your bladder fills up the muscles around it will contract to keep urine from leaking out until you’re ready – just make sure you can get out of your suit fast enough!

The dangers of holding your pee are mostly cumulative, so the occasional episode probably isn’t harmful. However, if you are diving frequently and often find yourself ignoring the need to pee you run the risk of urinary tract infections, urinary retention (the muscles can’t relax even when you want to pee) or bladder atrophy (leading to incontinence). But for most people, you can hold your bladder full for a few hours without serious complications, even though its uncomfortable.

If you dive in a wetsuit, you can of course make the call as to whether you pee in your suit or not. The old saying is that 50% of divers pee in their wetsuits and the other 50% are liars. So whichever camp you fall into, just make sure you flush your suit through before you get out of the water and start to take it off, and please wash your suit thoroughly between dives.  If you’re a drysuit diver, there’s always the option of adult nappies to ensure that you can relieve yourself. If that seems a little retrograde, or as you march middle age a little too prophetic, then perhaps a P-valve is an option.

Not surprisingly there are hazards associated with P-valves too. Ignoring the issues with getting a stick-on condom or the female cup attached successfully, so that the urine does actually enter the tubing to leave suit, there are reported cases of urinary sepsis. The tubing used to connect the urine to the P-valve is the ideal breeding ground for Pseudomonas bacteria, and it only takes a small amount of backward flow to introduce those bacteria into the body. If you think rinsing your wetsuit is a bit of a faff, syringing antiseptic through P-valve tubing should give you some perspective.

We’ve had a rather enjoyable club trip away during the winter.  A chance to escape the grey, wet and cold and get a bit of sunshine. It helped that the trip was planned as part of an Advanced Diver exped, as that meant I didn’t actually have to do much to smooth the whole trip along. Our AD candidate had picked a BSAC centre to go diving with, assuming this would mean that our reasonable levels of experience (including First Class Divers and instructors) would be recognised and we would be treated accordingly. The dive plan was submitted by email and when we arrived the owner of the dive centre went to quite some lengths to tell us he was going to follow our plan exactly.

Day 1 wasn’t so bad, apart from a really strange conversation about compasses, and how if you look at your compass and it says 90 degrees then you are in fact swimming West. We tried to explore this assertion, after all it would mean rethinking the entire way we brief for dive sites, skipper boats and well, just generally everything. How could we have all got this far and been 180 degrees out? We asked if this was caused by having a compass with a sighting window, maybe looking at it from the top, but nope.  We were assured that 90 degrees was West. End of discussion, let’s go diving.  We took our own compass bearings on the shore (safe bearing was North). Our check out dive took a strange turn when instead of the 15 metre bimble we’d been briefed for we ended up at a 32 metre wreck. During the surface interval, we were assured that this dive had been in the plan, but even the AD candidate looked puzzled and it was her plan! Whilst on the surface we asked about the time for the second dive. It was a mandatory hour away, but who knows why it’s an hour. A couple of us tried to look at our plan functions to gauge when we’d have enough dive time for the next dive but we were stopped because “decompression is only a theory anyway”. We were doing an hour on the surface and then we’d be going back in.

By the next day, things took another turn for the bizarre. The first dive brief of the day explained that the dive 1 would be out from the harbour and turn right, and the second dive at the same site would be out and turn left. The first dive was pretty good, shoals of fish and a couple of angel sharks. We came back to the shore, had a bite to eat and swapped cylinders. Whilst we were standing in the sun to warm up a bit, someone asked about the topography for the second dive. At this point the story changed, we were told that there was nothing to the left apart from barren rocks, our dive guide couldn’t imagine why anyone would want to turn left, or who had suggested we should and we would be going right again. The second dive to exactly the same site was disappointing.  The tide had changed, the shoals of fish had moved on and we resorted to a little bit of wombling to clean up the reef.

By now we had established that we were becoming the victims of gaslighting. Gaslighting is a form of persistent manipulation and brainwashing that causes the victim to doubt her or himself, and ultimately lose her or his own sense of perception, identity, and self-worth. The term is derived from the 1938 stage play Gas Light, in which a husband tries to convince his wife that she’s insane by causing her to question herself and her reality, flickering the gas lights as he does. Despite having been around for over 75 years, the term has recently resurfaced with Trump’s presidency in the USA.
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We had the classic signs; blatant lies, denial of what had been previously agreed and attempts to manipulate and divide opinion the group. Once we’d identified that this sort of behaviour was in play, everyone’s guard was up. Fortunately, we’ve all know each other for a long time and dived together for many years. Each night our dinner and beer debrief covered very little about the dives and much more about the bizarre antics. The carefully written up dive plan that was the point of the trip had disappeared and was never mentioned again. Next time we’re all going to put our basic qualification and not mention any further diving experience. We think it will be easier all round. 

Before 1920, having tanned skin was associated with working outdoors and indicated that you carried out manual labour. The societal ideal was pale skin which showed that you were rich enough to stay indoors, although the pursuit of pale skin often involved powders containing lead and mercury, which weren’t exactly great choices from a health perspective.  The Western Europe attitudes started to change in the 1920s when Coco Chanel started to popularise the idea of a tanned skin, and this trend accelerated through the explosion in cheap air travel from the 1960s, so that having a tan showed you could afford to take the time off work and lounge around catching some sun’s rays.

The concept of desirable skin colour varies around the world. Whites try to tan, whilst across Asia the sale of skin lightening creams and lotions is at an all time high. Your natural skin tone is determined by genetics and that is directly correlated to how much UV radiation you are exposed to. The higher the level of UV radiation, the darker the tone of indigenous skin. As some populations of humans moved to more northern latitudes there was a shift in genetics. With less UV radiation causing damage, there was a positive selection of individuals with lighter skin, who could synthesise more vitamin D. As societies changed from hunting to agriculture, there was a need to maximise the vitamin D synthesis to make up for the loss from the diet. About 40,000 years ago the mutations for pale skin emerged in both the East Asian and Western European populations.

The most important pigment in the skin is called Melanin. Specialised cells call Melanocytes make the pigment and pack it into the Keratinoctyes that make up a layer in the skin.  Melanin is an important molecule as it controls the amount of UV radiation that can penetrate the skin. Some UV radiation is needed for vitamin D synthesis, but too much is harmful. UV light is divided into 3 different wavelengths of light, UV-A, UV-B and UV-C.  UV-C and some UV-B are absorbed by the ozone layer in the atmosphere, which is a very good thing as without this absorption UV levels would be dangerously high (and quite possibly life on earth wouldn’t exist!). Some UV-B is absorbed by the epidermis (the upper layer of the skin but UV-A can penetrate further into the skin and interacts with the cells in the dermis. UV-B causes an increase in melanin production and UV-A causes the melanin molecules to change and become darker.

However, tanning is not the only consequence of UV exposure. Although no-one knew it during the 1960s, too much exposure to sunlight is the cause of 90% of all skin cancers, eye damage, immune system suppression and all the signs of ageing associated skin damage. As this message started to become understood, the tide has turned against tanning salons, and anyone out in the sun was urged to wear a hat, cover up exposed skin and slap on the sun cream….and then we hit another snag. In March 2018, Hawaii announced that it was bringing in laws to ban sun creams containing oxybenzone, which is the most widely used UV absorbing molecule in all sun creams in use today.

Once upon a time, Para-amino benzoic acid (PABA) was widely used in suncreams.  Patented in the 1940s PABA was the first molecule to be used absorb UV-B in sun creams, but it fell out of favour as it stained clothes and caused allergic reactions. Then came Oxybenzone as the next generation molecule with the ability to absorb UV-A and UV-B. Oxybenzone isn’t just used in sun cream, but as a UV protection for a wide range of plastics too.  Hawaii have recognised a study that showed that tiny amounts (microgrammes per litre) of oxybenzone cause coral larvae to stop moving around and prevented them from developing a hard skeleton. To understand the concept of just how toxic this is, that’s lethal levels at half a teaspoon in an Olympic sized swimming pool.
It’s time for a switch away from the UV absorbing molecules like PABA (still in use as the derivative padimate O) and oxybenzone. Perhaps we need to return to the mineral reflective suncreams with their chalky finish. Have a look at the label on your suncream. There are several alternatives hitting the market now, although for some of them the claims can be hard to verify. Perhaps the safest option for us and the environment would be to take the lesson from Victorian society and just cover up? Stop putting any plastic solutions into the sea including the lotions you slather on.

​In the depths of winter, there are two major factors that reduce diving time, low pressure weather systems and snot. As the air becomes colder and drier, the cells lining the nasal cavity have to work quite hard to warm and moisten the air that we breathe in. The cells producing the mucus are called goblet cells (which is a reference to their shape, not an instruction for what to do with the mucus). The mucus itself is a mix of proteins which contribute to the protective role in a number of ways; enzymes that can attack bacterial cell walls, antibodies to bind to pathogens and lactoferrin to mop up any free iron.
But the real star of the snot show is Mucin, a group of large proteins with lots of sugar molecules bound to the central regions of the molecule. These sugars are important as they allow the Mucins to have gel-like properties with an amazing water holding capacity. Aggregations of Mucin molecules are secreted by the cells lining the airways (and digestive tract too) and the sugar coating helps them to resist digestion. Over 20 human Mucin genes have been identified and the proteins that they produce help bind pathogens together, and are one of the reasons why you will make more snot when combatting a nasal infection.
It’s not just humans and other mammals that can make Mucin, a similar group of proteins is found in the most humble gastropods. We are all familiar with snail trails. (I’m sure that was my Nan’s phrase for a small child with streams of nasal mucus running down their top lip!) Snails move using a combination of their muscular foot and a lubricating slime. Now here’s where it starts to get strange, mollusc slime is a non-Newtonian fluid. It doesn’t follow the normal rules that govern viscosity in fluids, but rather changes as stress is applied to it. This explains why the same mucus can be used to allow snails to move and to bind to a surface. As the wave of contraction from the muscular foot of the snail acts on the sticky slime, the slime changes to become a free-flowing liquid. When the pressure is removed, the slime becomes gel-like again, allowing snails to lodge in overhangs and defy gravity.
For marine snails, it’s slightly harder to see the need for a lubricant, but it turns out that the slime trail for some species has even more functions. It’s a big commitment for some species to make a slime trail, estimated at up to 60% of their total energy use. Periwinkles will sniff out and follow fresh trails made by other molluscs to reduce this energy requirement. Mucus trails bind microalgae from the water when they are fresh and so they can be an excellent food source. Yep, that’s right, eating the algae from someone else’s snot trail is a good thing for Periwinkles, but please don’t try this at home!
Limpets are grazing feeders who return to their ‘home-scar’ on the rock every time the tide goes out. For them, the mucus trail is their route to find the carefully etched out rock into which their shell can clamp down to protect them from predators, sealed with a mucus layer to prevent them drying out. Not so much “Follow the yellow brick road” as “Follow the limpet snot trail” to get home. With the right conditions, you can see limpet snot trails on rocks as the tide falls.
For some molluscs, their slime trail is also important for mating. Chemical signals indicate the sex of the snail, allowing prospective mates to find and copulate. Male periwinkles can track down a female by following chemical markers in the slime. But the females of one species of periwinkle (Littorina saxatilis) turns off this signal to avoid mating. L.saxatilis live in dense colonies and like other periwinkles will mate up to 20 times a day throughout the year. This seems like a strange strategy for any species to survive, the general rule being that males mate as often as possible, whereas females try to be selective about mates.
Why would female L.saxatilis try to avoid mating? Males mount onto their mate and crawl around to the lip of the shell. This means that the female is then bearing the load of adhering both parties to the rock, and remember that our slime is non-Newtonian, more stress makes it flow. Having a male periwinkle on your back will double the stress and can result in both parties being swept off the safety of the rockline. For females, mating will increase their chances of being predated upon. So the female L.saxatilis turns off the sex signal in her slime. Males will still follow the slime trails, but it’s a 50:50 chance that they could be trying to mate with another male at the end of the journey.
Since Ancient Greece snail slime has been used in cosmetics. It contains high levels of hyaluronan which is a major component of the proteins that support our cells. It is freely available as a cosmetic claimed to promote the formation of collagen and help to improve skin structure. More seriously, hyaluronan is gaining popularity as a biomaterial scaffold which is helping the next generation of bioengineers to promote the formation of blood vessels in tissue engineering. Something to ponder when you are relegated to shore cover as you are too snotty to dive… 

​Normally the ferries coming to the Isle of Man run at sensible times, but there is one particular scheduled service that leaves the port of Heysham at 02.15. In the winter, when the only other crossing is 14.15, I seem to find myself on the ‘overnight’ boat far more often than I would like. The boat doesn’t load until at least 01.30, so for a couple of hours I usually try to sleep in the carpark. Cold, rainy and situated next to the nuclear power station, it’s not exactly conducive to any restful sleep. Even if I do doze off I still have that dreadful anticipation of being woken by the port staff to drive onto the car deck.
I’ve learned now to book one of the cabins on the ferry. Head to the customer services desk, collect the key and find the cabin with the beds made up ready. If I’m quick I can kick my shoes off and be asleep before the safety announcement. The journey is just under 4 hours and the arrival in the Isle of Man is accompanied by an announcement and the lights in the cabin coming on. It doesn’t feel like I’ve actually slept at all. After a short drive home, I usually try for more sleep, but it’s not always easy during the day. I usually need a good night’s sleep to recover from my acute sleep deprivation.
As divers we often travel some distance by road, ferry or plane to get to our dive destinations. Travel arrangements can involve early check-ins and sleeping in unfamiliar places. There is considerable research into the effect of sleep deprivation and its effect on behaviour, particularly for in relation to driving. Sleep deprivation has the same hazardous effect as being drunk. Research has shown that being awake for 17-19 hours impairs performance to an extent that is comparable to having a blood alcohol level over the drink driving limit for the UK. As drink-driving has now become socially unacceptable, how many of us are aware that our driving could be as impaired by lack of sleep?
I think back to my days living in London, getting up at 4am to tow the club boat to the South coast, two waves of two dives and some food followed by the drive home. The boat would be stowed away by about 10pm, so the last few hours of towing a rib would have had me well into the fatigue zone.  The evidence suggests that performance decline sets in after 16 hours awake, add this to sub-clinical decompression related post-dive tiredness and I think I was in dangerous territory.
How many times though do our trip risk assessments include fatigue? I got up at 5am this morning to collect a group coming in from the ferry. During the summer there is an 03.00 crossing from Liverpool arriving in the Isle of Man at 05.45. If I think I felt tired as I arrived at the ferry terminal – you should have seen the divers we collected! Some of them had managed a little sleep in the airline style seats, but not much. We’ve brought them back to the accommodation and sent them all to bed. We expect to be diving this afternoon, and one of the risks I’m now assessing is how much sleep they haven’t had.
I can’t find any specific research into the impact of fatigue on diving, but I am happy to accept that driving is a reasonably good surrogate activity. Drowsy drivers experience difficulty remembering the last bit of road and slower reaction times. Impaired cognitive and motor performance aren’t good for divers either. We learn about the impairment due to narcosis (with that amazing slide that has several pints of beer on!), but being awake for long periods is going to cause those effects without even stepping in the water. Maybe there are hints about this in our training, we do advise to have a good interval between flying and diving, but there’s nothing explicit regarding sleep deprivation. If you aren’t convinced that this is a problem, perhaps you should know that it’s been estimated that sleep deprivation is implicated in 1 in 5 road accidents. Sleep deprived drivers are much more likely to get angry with other road users and deal poorly with stressful situations (like navigating unfamiliar roads).
Caffeine can help, but only in the short term and not with all the aspects. It can improve alertness and reduce reaction time, but fine motor control isn’t improved even with high doses. So, I could send all the divers to the local coffee shop and insist they top up their espresso quota, but I know that won’t last. Instead, I hope they have their heads down and are napping now. Me? I’m too wide awake and writing columns instead!

​Sellafield is located across the Irish Sea on the Cumbrian coast and is approximately 32 miles from the Isle of Man, on a clear day you can just about see it. The main activities at the plant include reprocessing of spent fuel from nuclear power reactors and storage of nuclear waste. There are no longer any nuclear power plants in operation at the Sellafield site. It was built in the late 1940s to manufacture plutonium for atomic bombs and Sellafield is one of the most radioactive places on earth. In its prime the plant was releasing eight million litres of contaminated waste into the sea every day. In 1957 the plant became the site of the worst nuclear accident in Great Britain's history, The Windscale Fire. This was a blaze that raged for three days, releasing radioactive gases into the air. The discharge of low level liquid wastes from the Sellafield site in the north west of England is the most significant source of artificial radioactivity in the Irish marine environment.
Now the site is mainly used for nuclear fuel reprocessing, and this and other activities gives rise to the discharge of low level radioactive materials in the form of liquids and gases into the environment. These discharges are regulated by the UK authorities and limits for releases are set by the Environment Agency of England and Wales (EA). Liquid radioactive waste is discharged from the plant into the Irish Sea via a pipeline, about 3 km from land. Gases are released from the plant via a number of chimneys (referred to as ‘stacks’). Discharges into the Irish Sea peaked in the mid-1970s and have dropped significantly in recent years. This is as a result of improved waste treatment facilities at Sellafield, which convert much of this radioactive waste into a solid for long-term storage.
As a result of the discharges from Sellafield, low levels of artificial radioactivity can be detected in sediments, seawater, seaweeds, fish and shellfish taken from the Irish Sea. A wide range of marine samples are collected and analysed on a regular basis by both the EA and the Manx Government. This monitoring can show where the radioactive particles become concentrated.  As expected many particles end up in sea bed sediment, so there are sometimes slight increases when the winter storms have been especially ferocious and stirred up the seabed. Generally, levels are falling from their peak in 1998. 
There are several radioactive isotopes that are monitored, Technetium-99, Caesium-137 and 134 and Cobalt-60. Of these, Tc-99 is regularly tested for by catching lobsters. Tc-99 concentrations in our local lobsters have declined from a peak of around 400Bqkg-1 in February 1998 to average 10 Bqkg-1 during 2015. These Tc-99 concentrations are lower than the levels found in lobsters caught off the Cumbrian coast. The EC recommended maximum permitted level for Tc-99 in seafood which is 1250 Bqkg-1, so these lobsters are safe to eat and regularly eating seafish will only make a minor contribution to your overall radiation exposure.
Now it’s not true to say that lobsters are immortal, but once they reach adulthood they don’t have many predators except humans. Good lobster fishery management sets minimum landing sizes for lobsters, ensuring that they are at least able to breed once before being caught. Small lobsters can get out of pots through the escape hatch or they are returned to the sea anyway.
Just as lobster pots discriminate against small lobsters, they also prevent very large lobsters from getting in.  Consequently, larger lobsters do tend to live a very long time.  The lifespan of European lobsters has been estimated at between 30 and 50 years.  Large lobsters have lived through the peak discharges from Sellafield, unlike their smaller 3-4 year old counterparts who got caught in lobster pots and tested. Lobsters have a fairly high affinity for Tc99 and they accumulate the radioactive particles in their bodies. But the only real predator for the large lobsters is, you’ve guessed it, divers. 
Something to think about the next time you wrestle a monster lobbie from under a rock

​Somewhere back in your very first diving course there will have been reference to air being compressible because it’s a gas, as opposed to water being non-compressible. When we start thinking about how molecules interact, it’s helpful to go back to school science. In solids the molecules are packed tightly together, they can vibrate but they aren’t free to move around much. In liquids the molecules can move but stay interacting with each other and in gases the molecules fly around freely bouncing off the walls of whatever contains them.
Thinking about gas molecules as random whizzing around bumping into other molecules and their container, it’s easy to understand why if we push more molecules into the container the number of collisions with the container wall will increase and the pressure goes up. This kinetic model of gases assumes that the gas particles themselves are very small (they are) and there’s a lot of space between these gas particles. That leaves lots of extra space for us to jam more molecules in there and compress the gas together. Hence gases are compressible.  This nicely explains Boyle’s Law (remember that?) but makes a number of assumptions which create a concept of an ‘Ideal gas’. Sadly, Ideal gases don’t exist and we have to deal with real gases. But at low pressures Boyle’s Law is fairly useful.
Liquids don’t behave in the same way because the molecules are already close together and interacting to a limited extent. Each molecule forms temporary associations with the other liquid molecules. If you heat up a solid to melt it into a liquid, you can measure the temperature increasing. As you get to the point where the solid it melts, the temperature will stabilise (even though you are still heating it). This is the point where the molecules are absorbing heat energy to give them the kinetic (movement) energy to move around. Once all the solid molecules have absorbed enough energy to melt into liquid, then you can see the temperature start to rise again as you carry on heating it. During this phase the molecules will move faster, but they still interact with each other. Keep heating and give them enough energy and they will manage to escape the interactions and form a gas.
Water is a bit of a strange liquid, because it’s molecules interact more than other liquids, and this gives it some strange properties. Water molecules form hydrogen bonds to other water molecules, and then they break these bonds and reform them with other nearby water molecules. Although these bonds are quite weak, there are lots of them. If it weren’t for hydrogen bonds then water wouldn’t be a liquid at all. When the other elements in the same family of the periodic table, like Sulfur and Tellurium, bind to hydrogen they form gases not liquids.
Water is most dense (the molecules are packed tighter together) at 4 degrees. At this temperature the hydrogen bonds are quite structured and pull the molecules tighter. As the temperature rises the bonds start to make and break more frequently and allow the water molecules to move a little further away from each other, so water becomes less dense. At temperatures below 4 degrees, hydrogen bonds don’t form so well and so as water cools to become a solid, it also becomes less dense. This explains why ice floats on liquid water (good news if you’re a polar bear) and why cold water sinks into the ocean (that’s  thermoclines).
So, these little fairly weak hydrogen bonds have a lot of influence and it’s them we are fighting against when we try to compress water. They have already done the job of pulling the oxygen dihydride molecules far closer together than we’d expect from the other elements in their group. It’s because of them that water isn’t a gas at room temperature.  And once we’ve got as far as forming a liquid, there isn’t much compression left to achieve. At 4km down in the ocean, water has a measurable compressibility of just 1.8%. It’s not quite true to say water isn’t compressible, it’s just not very compressible and for the depths that we will visit we can probably ignore the marginally increased density.
Resistance to compression for any substance can be described by the bulk modulus value. This is a measure of how much pressure must be applied to reduce the volume by 1%. For solids, these values are predictably very high, eg diamond is 443,000,000,000 Pascals and steel is 160,000,000,000 Pa. For ice I would need 2,000,000,000 Pa to compress it by 1%.  For liquids we would expect the values are lower and generally they are. Water bucks the trend though and needs 2,200,000,000 Pa. So, I actually need more pressure to compress water than I do to compress ice. Blooming pesky things those hydrogen bonds!

​At the end of 2016, diver training agencies including BSAC launched Sea Survival training courses developed in conjunction with the RNLI. Despite the common view of the RNLI as being the provision of lifeboats and crew, there is much more that they are involved in from a safety at sea perspective, with particular foci on fishing industry accidents, Swim Safe training courses and safety advice for all water users. It’s astounding how many fishermen don’t wear lifejackets, especially local pot-boat skippers who often work alone.
To try to educate fishermen the RNLI brought a dozen of them from around the UK down to their training base in Poole. All the skippers had previously attended the mandatory Personal Survival Techniques course (and it’s predecessors) which are run in swimming pools around the country. The RNLI trainers asked about lifejackets and got the usual story, the fishermen had them but rarely wore them. The general feedback was that as strong swimmers they were confident that should they fall in the sea, they would be able to swim back to their boat, climb up the tyres on the side and self-rescue. Interestingly, qualified divers and anyone who swam in the sea was excluded from the test group. Repeated attendance at sea survival training had led each fisherman to conclude that their lifejackets weren’t necessary.  The RNLI sought to challenge that belief.
The night before the training course, the trainers opened the doors around the training pool to let out the heat. Overnight the water temperature dropped to 15 degrees.  If you are a diver around the British Isle I am sure there are days where you dream of 15 degree water! At the first attempt the fishermen were asked to wear their normal deck attire and jump in to deep water to simulate falling off their boat. With no life jackets on, the impact of cold water shock was immediate. None of the 12 fishermen lasted longer than 5 minutes before a rescuer intervened. Post dip interviews revealed their shock and surprise at how debilitating the cold water was, definitely nothing like their sea survival training course.
Cold water shock is an immediate short-lived response to immersion in water less than 15 degrees. Blood vessels at the skin contract rapidly, increasing blood pressure and the heart rate. An initial gasp for air can be followed by a breathing rate that is 6-10x higher than normal. It is likely that cold water shock accounts for most deaths when people have unexpectedly entered the water. If you are not wearing flotation during this phase, keeping your head above water becomes the biggest problem.  Over the next 10 minutes, cold incapacitation reduces blood supply to the muscles, making it difficult to swim or self-rescue. A crew member throwing a life ring to you during this time will be frustrated that you can’t actually hold onto it or kick towards the safety of the vessel.
The following day the exercise was repeated but this time with lifejackets being worn. The same cold water shock reaction was initiated, but the fishermen didn’t have to work so hard to keep their airway out of the water, the cold incapacitation stage took longer therefore improving their chances of getting back to the ladder on their boat. You can see the videos from this exercise on the RNLI website.
This started me thinking about why divers were excluded from the test group. I’ve realised I still brace myself for the cold water after decades of diving. OK, I’m wearing a drysuit and the cold water shock reaction is pretty much limited to my head and hands. But how many of us drop beneath the surface in anticipation of that brain freeze moment? As the blood vessels rapidly contract they stimulate the trigeminal nerve sending pain signals to your brain. It hurts for a few moments until you become acclimatised. The fishermen in the RNLI training exercise couldn’t get past that brain freeze feeling.
I think we sometimes underestimate the impact that cold water immersion has on new divers. I can recognise it enough now, but when I think back to learning in a wetsuit I can remember the feeling of panic, rising heart rate and accelerated breathing rate as I used to get into the water. Although we will all recognise increased air consumption by trainee divers, perhaps part of this is their reaction to cold water immersion? I’m sure that with experience comes the anticipation, the forced control of breathing rate for the first few seconds, but until our new divers have developed their response, maybe we should keep a close eye on them for those first couple of minutes? If your trainees are hoofing through their air and their buoyancy is being disrupted by their rapid breathing rate, maybe it’s something to consider?