Anyone who intends to dive in British waters needs a drysuit. True, you will see the odd diver wearing a semi-dry, or even a wetsuit, but the only appropriate action this should prompt is to take these poor unfortunates gently by the hand and lead them indoors. There they can be given a warm cup of tea and a biscuit while you try to establish the name of the nursing establishment from which they have absconded.
And why are drysuits so important Because theyre uncomfortable to wear, take time and effort to learn to use, and look really distinctive. This immediately marks you out as a proper diver, and not some warmwater, good-viz, lots-of-interesting-and-colourful-stuff-to-look-at holiday tauchmeister.
Drysuits come in many designs and colours, and can be made from many different materials, but there are only two types with which you need to concern yourself: those that keep the water out, and those that dont.
The former are a wonderful, yet temporary, phenomenon. Sooner or later all drysuits become dampsuits if youre lucky, and wetsuits if youre not. True, warmwater divers call a tightly fitting drysuit without seals a wetsuit anyway, but they also call 20m of viz limited, so what do they know
Drysuits are made from one of two basic materials, membrane and neoprene. Membrane drysuits are made of a thin, tough material that bends and shapes to forms quite well, but doesnt stretch much. This means that they need to be generously tailored to permit freedom of movement while the suit is being worn, and will therefore be what is technically known as baggy.
This looks just awful on the dive-boat, where you will resemble nothing so much as a badly tied sack of ferrets.
Once under water, the increased pressure will force the suit to mould very closely to your personal contours. You will then look as if somebody has stunned the ferrets and vacuum-packed them. There is no point in paying extra for a made-to- measure membrane suit, because you wont be able to tell.
You do need a suit that fits your height, however. Go head-down, feet-up in a membrane suit which is too big, and the bootees will be blown off the end of your legs, leaving you inverted, surface-bound, and with the finning power of an out-of-date lettuce.
Inland dive sites around the country are characterised by the waving feet of novices desperately trying to un-invert themselves before their air runs out. Deliberately inverting novices isnt big, isnt clever and isnt nice.

On the plus side, however, membrane materials are available in a wide variety of interesting and attractive colours, so you can have a suit made which is colourful, distinctive and expressive of your individual personality.
Faced with this opportunity, however, the average diver picks either a combination of colours so vile that the suit should be sold with protective eye-wear, or black. Very, very black. Divers know that the tekkies like black, and a black suit suggests technical diving credentials, and thus becomes a potential status symbol in a sport otherwise devoid of the opportunity to compete.
Neoprene drysuits can be made from ordinary neoprene, compressed neoprene or crushed neoprene, and will be made to fit much more closely out of the water because neoprene stretches as you move. Allegedly.
On the other hand, and there is always another other hand in diving, neoprene drysuit wearers do tend to look and walk like the sort of people who regularly abuse anabolic steroids. This is very useful for macho swaggering on the jetty.
Ordinary neoprene is around 8mm thick, and really is stretchy. It is an excellent insulator and very buoyant, so you stay warm, but need a lot of lead to get beneath the water.
Once under water the increased pressure compresses the neoprene, counteracting both the insulating effect and the positive buoyancy problem. This means that you spend most of your dive cold and over-weighted. On the bright side, however, the effort of swimming when you are so over-weighted does tend to keep you a bit warmer.
In an attempt to reduce these undesirable side-effects to a minimum, the industry developed compressed neoprene. This is neoprene that has been pre-compressed to simulate the effect of water pressure. This type of suit promises good thermal insulation and little buoyancy change at depth.
One out of two is not a good result. These suits tend to fit so closely that you cant wear too much underneath, so you spend your dive neutrally buoyant but cold.
And the outer layer of nylon bonded to the neoprene to make it abrasion-resistant and hard-wearing tends to hold water after the dive has finished, so the wind-chill keeps you cold well after surfacing. Especially in a RIB.

Crushed neoprene is neoprene that has been crushed to remove the insulation and prevent your buoyancy altering during the dive. It achieves both aims by making neoprene into a membrane material but, and this is important, at a much higher price.
As we all know, the more expensive the kit, the better the diver.
All drysuits have seals at the wrist and neck to keep out water. These can be made of latex or neoprene. Latex seals are made of thin latex and fit tightly around your neck and wrists to prevent the ingress of water. Theyll cut off your circulation and leave you with three perfect hangmans lovebites, but they do work.
Neoprene seals are tubes of neoprene a good few inches long, and work by fitting smoothly and closely to your skin over such a long distance that the water loses interest before you get wet. Neoprene seals are vastly more comfortable to wear but dont always keep the water out. The choice is yours.
hspace=5 Drysuits also need a tough, watertight zip so that you can get in and out of your suit. Its position is important. If it lies across the back of your shoulders - the so-called rear-entry position - the zip is relatively short and doesnt flex much in use, and should therefore be more reliable and last longer.
You will need a buddy to close this sort of zip, however, and buddies always give the zip a good tug at the end of the travel to make sure it really is closed. This stresses the zip and eventually rips the zipper off, thus negating the longevity advantage.
Front-entry zips run from one shoulder and across the body to the opposite hip. The advantage is that you can suit yourself up and see to your own zippage, so it should be more difficult to forget to do up the zip before you hit the water.
At least one enterprising manufacturer, however, opted to fit the watertight zip in the suit and then cover it with a protective flap secured by an ordinary, heavy-duty, non-watertight zip. The intention is not, of course, for the diver to enter the water with the outer zip fastened and the inner zip open. In addition, front-zips flex a lot in use and consequently can fail after a relatively short time.
The convenience zip is also worth a mention. This is a short horizontal zip at groin level and is intended for male divers to open for purposes of micturition. It can provide hours of fun for all the family.
The best part is watching the diver struggle to get the nasty sharp teeth of the zip and the nasty hard edges of the zip far enough apart to extract the relevant appendage, then hold them apart for the necessary duration while standing on the swaying, heaving and only theoretically non-slip floor of the RIB.
Most divers are so happy that the process has been completed without self-inflicted surgery that they dont remember to zip up afterwards. And you can be sure that nobody will tell them.

Which brings us finally to valves. Drysuits trap air at the surface, and therefore at surface pressure. As you descend, the air is compressed by the increasing water pressure, reducing both buoyancy and insulation. If gas isnt pumped into the suit, the buoyancy changes can become critical, and the only way is down.
A pre-dive meal rich in carbohydrates can be of some use, but most people prefer to use the gas from their diving cylinder, which can be connected to a suitable inlet valve via a sturdy low-pressure hose.
Inlet valves should be situated on the front of the suit, preferably somewhere easy to reach and not covered by the straps of your BC. They allow air into your suit either when you press the button on the valve, or when they jam open. Always check the roofs of wrecks or caverns for divers suffering from irreversible inlet-valve problems.
As well as counteracting changes in buoyancy, adding air to the suit makes diving more comfortable. Just ask young Robert, who managed half a dozen open-water dives before his mum noticed the bruising on his arms, legs and chest.
Air is also an insulator, and many divers will add a few pounds to their weightbelts in cold water just so that they can put a bit more air in their suits to help them stay warm.
This is achieved by a combination of effects. First, you must work harder to carry the extra weight around, so youre warmer when you hit the water. Second, you have to work harder in the water to deal with the extra weight, so you stay warmer during the dive. Third, and most important, all this extra work means that you use your air faster, so you can come up sooner and get into the pub earlier, where it really is warmer.

Technical divers often carry a small cylinder of argon for suit inflation. This has the triple benefits of increasing kit complexity, adding cost and looking dead flash.
Getting air back out of the suit is essential. Ask the chap I saw come up from the bottom of Hodge Close so fast that his fins cleared the water surface. Modern dump valves are automatic, and open to vent excess air as soon as the pressure inside the suit is greater than the pressure outside. Unless, that is, you are already ascending at a dangerous rate, when they will often jam shut.
In the olden days, dump valves were situated on the forearm and vented air whenever the arm was lifted, conclusively demonstrating that the inventor of the cuff-dump had never dived.
Generations of divers could be seen swimming around with one arm as far below their body as they were able to reach. At the time we didnt think it odd, and just assumed that they were trying to get their depth gauges to read the maximum possible in preparation for the post-dive pub bragging session.
And finally, undersuits, which are essential if you are to be comfortable diving in UK waters. Proper undersuits with designer labels are available for membrane divers, but anything you can cram on does for neoprene divers.
Just bear in mind that all undersuits are drysuits when you kit up, and wetsuits after the warm-up exercise of humping the boat down the beach and launching the damn thing.