've enjoyed all that has been said on the topic of wet sutis for sailing....
However, I thought I'd also suggest that one should dress for the water...not only for the air temperatures.
I do a lot of sailing and have found: if I don't dress for the water and air temps, life can get pretty miserable.
My set up, in cold water is a full wet suit (farmer john and top.)... with a semi-dry spray top. My wet suit top (a shorty) opens with a diagonal zipper, so I can open it if I get warm.
I've also noticed it is tempting to dress down on sunny days, forgetting just how cold the water is...and feeling that if I dumped the boat....I could get it righted and me out of the water in pretty short order..... HOWEVER....!!!!
In those unexpected conditions.... if one ended up separated from the boat or if the boat went turtle and one had to ride it out for an extended period..... and if one wasn't dress for it..... well..... I've read about how some folks have died of exposure.....because they were too optimistic or under prepared or such....
Also, keep in mind how hard it is to put a wet suit on in the water on the tramp of a bucking boat....
Be prepared and stay prepared for the worst...not the best.....
Just a thought....
Tom H.