The best is to be like the sailing Pro's, who wear $700 drysuits.
Cheapskates like me wear 5mm wetsuits when the water temp is cool to cold. Up here in Canada, when the wind chill hits, I add a top layer of nylon windbreakers, (pants and jacket, such as K-Ways, Mountain Coop,whatever).
When it gets even colder, I first put on a pure wool sweater. Our sailing ancestors knew that the advantage of wool is that it stays warm, even when wet. My most recent experience was on Oct 22, Air temp 12C/54F, water temp 15.5C/60F, when the trapeze hook became undone and I took an unexpected swim. Yes, it took us an hour to reach back to the club, and by that time, my feet and legs were cold. Any more sailing like this, and I'll spend the cash for better gear. Or, maybe I can get a sponsor.
1989 Hobie SX18 Sail # 1947
'Only two things are infinite, the universe, and human stupidity. But I'm not sure about the former.'