Originally Posted by alienator .
The water between you and your wetsuit does provide an effective thermal barrier, which is also what insulation does/is. Dry suits are used for dives in very cold water or extended dives in cold water. They are used because at the temperatures at which they're used, your body could not transfer enough heat to the water in a wetsuit, over time, to sustain a safe body temperature. It's just a thermodynamic reality. It's all a function of body surface area, heat transfer rates, and finite energy source the body has.