Drysuits are the preferred exposure suit when diving in cooler waters or when the extended bottom time is desired. They allow divers to extend dive times substantially compared to wearing a wetsuit or semi-drysuit. New designs, materials, technology, and undergarments mean that they are now commonly used in warmer waters (20° Celsius/68° Fahrenheit or more) as well.
As previously stated, wetsuits allow a layer of water to enter, and body heat warms the water. In contrast, drysuits do not allow water to enter. The suit itself provides minimal insulation, and most of the thermal protection is provided by wearing clothing called undergarments under the suit. The colder the water, the warmer the garments need to be.
Drysuits use very specialized dry zippers (generally across the diver’s shoulders or across his or her front) that allow the wearer to get into and out of the suit. These zippers are similar to those used on an astronaut’s spacesuit and are airtight and watertight. They are key to the function of the suit and demand particular care and lubrication with a compound of beeswax or another semi-solid compound. Modern suits use either steel or plastic zips. In order for a drysuit to truly stay dry, every seal must be perfect. Therefore, the suit is fitted with dry seals for the diver’s wrist and neck. These seals are made of latex, neoprene, or silicone. Drysuits have built-in boots or neoprene socks and over-boots.
To compensate for the increase in pressure as the diver descends, air must be added to the suit to prevent it from squeezing the diver. Suit squeeze is extremely uncomfortable because the lack of air inside the suit makes it cling to the diver’s body. In cases of extreme suit squeeze, a diver can suffer bruises and painful welts. Therefore, drysuits have an inflator valve (much like those found on a BCD) on the front of the suit, which allows the addition of air or gas to alleviate suit squeeze. A low-pressure hose, fed from the first stage on the cylinder, allows divers to add gas as they travel deeper. On ascent, the gas in the suit will expand. Therefore, the suit has a dump valve, which allows the expanding gas to be released, enabling a safe controlled ascent.
Using a drysuit is quite simple but does require specific training. This training can be combined with most training programs—especially those conducted in cooler climates (e.g., northern Europe, Canada, parts of the US and Asia)—including Open Water 20, or as a separate specialty program.