The major reason why a suit is necessary for scuba diving or any other related activity is that it is a much better conductor of heat than air. On land, walking in under 70 degrees feels nice but swimming in a 70-degree water isn’t so nice. The reason being that heat removes heat from the human body. About 20 times faster than air. This happens because the body heats up the thin layer of water surrounding it and the water expands, moves up and is replaced by cold water. This cycle continues until the chills begin to set in and happens even in water which most consider warm, say 80 degrees. For a human to feel comfortable in water for an extended period, its temperature would have to be up to 90 degrees, and the primary reason for this is that our body temperature is just under 100-degrees Fahrenheit. If the water is any cooler, heat is lost, and this happens faster with increased temperature differential between the human body and water. Other reasons why divers wear exposure suits are that it protects against cuts, abrasions, stings, infections and other things which could happen to bare skin.
Primarily, there are three different types of exposure suits: dive skins, wetsuits, and dry suits. Each kind comes in different varieties depending on the kind of environment.
Dive skins are the lightest, thinnest exposure suits and they offer the least amount of protection. They are about half a millimetre thick. They consist of either Lycra, thin neoprene or Polartec. Dive skins are highly elastic, body-fitting and come in several colours and colour combinations.
Design wise, dive skins have a zipper up the front, a loop that goes around the thumb to keep the sleeves in place and stirrups to keep the pant from riding up. Dive suits give protection from corals, blisters, and sunburn when you get out of the water and they provide some warmth. Although the warmth which it provides is limited as water penetrates the suit. If you need a dive skin that keeps you warm, choose one that has a fleece lining or go for warm-water suits with thermoplastic insulation between two layers of spandex. Thermoplastic insulated suits have the added benefit of being windproof, and thus warmer out of the water under wind exposure. They are also worn under wetsuits for extra insulation.
Dive suits are mostly worn in tropical waters with ambient water environments. They are also worn under wetsuits and in fact, wearing a dive skin makes wearing a thick wetsuit a whole lot easier.
Wetsuits are the most common exposure suits and are designed to slow down the rate water cools your body. The process which creates neoprene introduces air bubbles into the material, and when divers get into the water, the water gets absorbed into the outer layers of the wetsuit. There, the water gets trapped and acts as an insulation between the body and outside water, and in the process, the rate of heat loss reduces gradually. The thicker the neoprene, the higher the insulation factor and the more positively buoyant you become. The thickness one needs to keep warm varies by the individuals and activities they are involved with. As a general rule, a 3mm thick neoprene suit is ideal for water temperatures between 75-85 degrees. Reducing the temperature to around 65-75 degrees, the most common wetsuit are worn at 5mm thickness, and because the head is one of the major places where heat gets lost, most divers will use a hood at this temperature range. Below the 7 and 8mm suits may be employed and some divers will use an 8mm in the water down to the high 40s depending on the diver’s comfort level. You could get wetsuits in the thickness known as semi-dry suits which are designed with dry suit styled cuff around the hands, neck, and feet. This is done to restrict new water from entering the suit which your body would have to heat.
Dry suits are worn when the water gets too cold for your wetsuit. With a wetsuit, the thickness of the suit determines the temperature the diver will find suitable. For divers who prefer cold waters, the diver will wear thicker suits and a dry suit keeps water away from the body. You adjust for different temperatures the way you would do in winter, you remove or add layers of clothing under it. However, diving with a dry suit takes a little more training. The suits will have an inflator valve and an exhaust port. The dry suit then replaces the BCD as the primary method of buoyancy control.