Wearing a fully insulated dry suit is mandatory for taking the chopper out to some of the oil rigs in the Bass strait that I used to visit. They are usually a fully insulated rubber exterior, integral boots and hood with extremely tight collar and cuffs to prevent the entry of water. Underneath you wear a feather down unisuit for more insulation. They also have gloves stored in a zippered compartment in the hood to put on after entry to the water. Even then, in cold sea, they are only expected to keep you alive for 48 hours tops.
A standard thick wetsuit will keep you afloat, the only problem is floating you face up so that you still have an airway if you are unconscious, some sort of life jacket or flotation inserts are needed to 'right' the person and keep them from floating face down.
It reminds me of a friend we lost a few years ago. They went fishing off our south coast here in the middle of summer. There had been some people stealing small items from the back of boats so they took everything out of the boat the night before and stored it inside. When they re-packed the boat they forgot the wetsuits.
Out fishing they got swamped by a freak wave. The first two - a father and son (both very tall, lean men) - died within 8 hours. My friend, a super fit swimmer and triathlete, survived through the night but died the next morning within site of land, 18 hours after the sinking. The only survivor was a very overweight, unfit man in his late 40's. This was in cool, but not extremely cold water. If they had remembered their wetsuits (and been able to salvage them as the boat was sinking) they would have survived.
