Drysuit
As one of the other posters said, it is always a compromise having a suit which will keep you warm enough but be comfortable enough to wear.
I went for the Crewsaver Hyperdry from Wetsuit Outlet.
It has a front zip - which I think you DO want as you can undo it yourself in the cockpit after you have passed over water.
I tried it out in January, sitting in a freezing pond (~2deg C) for 45 mins, with the included fleece lining. Kept me warm enough except for my feet which got fairly cold. I wore 5mm divers wetsuit gloves and 7mm hood as well.
It is tight around the neck and wrists and keeps the water out.
It is not 'approved' for offshore use but it is at least wearable in the cockpit, does not affect using the aircraft controls and does not restrict movement (e.g. emergency egress).
I would say that wearing it would keep me alive in really cold water for some time; plus prevent the cold shock that would otherwise incapacitate (and wearing it would therefore give me a reasonable chance of getting in a life-raft).
It also traps some air and therefore provides some buoyancy on its own.
(And I have practised getting in a life-raft while wearing it, and it is OK.)
I have worn it on a 5 hour flight from Denmark to the UK in winter time (fully done up - a bit warm but OK); also on a flight from Exeter over the water due south to France - as it was hot I then unzipped it when over land again.