One more remark: For some reason - and this holds both for me and my friends - it seems that if you first go in and sail for something like 20 minutes and then go up and get you hands warm, then you can afterwards sail for much longer without getting cold. It's like a kind of tempering.
This is a very effective strategy. The problem with it when it's really cold is that your body as a whole might get cold as soon as you're on land, not keeping active. But some extra layers of clothing while on land will help with that.