Dave
I can remember Scuba Diving under a frozen lake. we dug a hole in the ice which was thick enough to stand on and all I was wearing was a full wet suit not a dry suit!
All was fine until the suit tore under the armpit the shock was horrendous and that was on a limited exposed body!
I do not know how they work out the surveil rate times and am sure some will last longer than others.
warm seas you have the problem of sharks fine as long as you stay reasonably still having done a Shark feed off the Bahamas I love the critters
and have met and swam with them many times as well as filming them so they do not hold the fear I would have of ditching in freezing water in the North Atlantic
Pace