Avoiding losing body heat

Water conducts heat from the diver 25 times[1] better than air, which can lead to hypothermia. Except in very warm water, the diver needs the thermal insulation provided by wetsuits and drysuits.

In the case of a wetsuit, the suit is designed to minimize heat loss. Wetsuits are generally made of neoprene that has small gas cells, generally nitrogen, trapped in it during the manufacturing process. The poor thermal conductivity of this expanded cell neoprene means that wetsuits reduce loss of body heat by conduction to the surrounding water. The neoprene in this case acts as an insulator.

The second way in which wetsuits reduce heat loss is to trap a thin layer of water between the diver's skin and the insulating suit itself. Body heat then heats the trapped water. Provided the wetsuit is reasonably well-sealed at all openings (neck, wrists, legs), this reduces water flow over the surface of the skin, reducing loss of body heat by convection, and therefore keeps the diver warm (this is the principle employed in the use of a "Semi-Dry")

In the case of a dry suit, it does exactly that: keeps a diver dry. The suit is sealed so that frigid water cannot penetrate the suit. Drysuit undergarments are often worn under a drysuit as well, and help to keep layers of air inside the suit for better thermal insulation.

Drysuits fall into two main categories neoprene and membrane; both systems have their good and bad points but generally they can be reduced to:

• Membrane: high level of diver manoeuverability due to the thinness of the material, however that also means that heavy weight undersuit is required if diving in cooler water.
• Neoprene: low level of diver manoeuverability due to the material being considerably thicker than membrane material (even when dealing with compressed neoprene) however the neoprene provides a higher level of insulation for the diver.

Avoiding skin cuts and grazes

Diving suits also help prevent the diver's skin being damaged by rough or sharp underwater objects, marine animals or coral.

information: Wikipedia