Hmmm - whilst all these comments are true - I don’t disagree at all - the best comments are form those who have done it - not the ditching I hasten to add, but been swimming in the winter.
Hats of to RatherbeFlying.
I have dinghy sailed for years and am a BSAC diver. When I first started sailing wet suites were relatively new and we couldn’t afford them any way.
I recall sailing 470 at Calshot in January with the sea water in the bottom of the dinghy just turning slushy. We had lots of layers on, and waterproofs, and we were fit and use to it. We "survived" capsizes and dragged ourselves onto the centreboard and got going again. I recall standing in the shower after one race and the water turning blood red - literally. A cut had warmed up unbeknown to me and started bleeding profusely. Time in the water during a capsize was short, we were fit and expecting it occasionally. The rescue boats were very close at hand. And the point is - well if you are focused, fit and deal with the situation as long as you can get yourself into a liferaft you stand a chance without a dry suite - but a dry suite / immersion suite makes a huge difference.
Some years later I recall two other people sailing a fireball - it was wet suites by then. A capsize followed by getting the mast stuck in the mud lead to around half an hour in the water. They were both girls with low body mass. One was nearly hypothermic after being dragged out the water by the rescue crew. Her account of how rapidly she became unable to do anything was sobering - as was the very pleasant haze into which she fell - as she said at least it was a nice way to go.
A few years on and nearly everyone who sails dinghies in the winter wear dry suites. They are actually pretty comfortable and even breathable. Of course their effectiveness ultimately depends on the layers of thermal insulation under neath (and remembering not to forget to close the zip if you have been wearing one in the cockpit). With good thermal under layers they are brilliant, floating in the water in January is a non event - albeit I have never done so for a great length of time so how long they ultimately give you I don’t know. In the water there will still be a significant heat loss through the head and hands, and feet if you do not have the fitted booties. Diving in a dry suite has similar advantages and “pressurised” they are more than adequate to keep you warm for the entirety of a diver.
I guess a large size would fit most people if needs be (thinking of carrying different pax) but most people just think it is bizarre to get into one of these before going flying. Moreover sitting in a warm cockpit with proper thermal wear underneath is not going to be one of the most comfortable experiences. I tried it once so I know. It is quite doable however and if you want to weight one more odd in your favour its not as bad as you might imagine.
In short it all boils down to risk assessment. I suspect if you want to have any chance of surviving for more than an hour in the water in the winter you need to be wearing a dry suite, as a second best you might get away with a raft if you are fit, know how to get into it and do so quickly - anything else .. .. ..