Hmm...I thought so too and got a surplus neoprene one off Ebay for £35 ages ago. It has integral mitts and boots with a tight fitting hood. To be honest, it takes a bit of getting into but if I was contemplating using it in anger I suspect that motivation levels would be high.
I also have a Mullion X4 suit for working in 'orrible weather on commercial vessels/rigs. I have never used it sailing but it would be very warm and dry in the winter. Its very much like the angler type floatation suits but in safety orange.
There are ex-RAF drysuit type immersion suits on ebay and the other non-neoprene membrane survival suits too. Both are likely to be easier to get into and I remember trying the latter once and it was much easier to don.
If I was venturing offshore I think that I'd have one with me + a liferaft as its exposure that is going to get you if you do have to get into the thing.
Lets face it though, what really is the risk of needing one when pottering around the coast? Anyone want to buy a use survival suit? :-)
Fifteen men on a dead man's chest
Yo ho ho and a bottle of rum