If you think about it, when you fall into cold water what's the biggest thing you have to worry about sinking or freezing to death?
If I had my choice between flotation and something that would protect me from hypothermia I'd take a wetsuit.
Speaking of that every year I am out in the water within a week of ice out putting in my dock, 3 hours in the ice cold water with a 7MM wetsuit really isn't all that bad. No it's certainly not Bermuda but sure as hell is more comfy than t-shirt, shorts and a PFD.