I just wanted to acknowledge your idea, which is intriguing to me. I doubt, however, that many boats of the size we SailNet members sail would be anchored in 50 feet of water very often. In the warm water venues we have visited, you''re usually in 10-15 feet of water, if you are lucky. With water that shallow, I don''t know what the water temperature would be relative to the night air temp in which you would be trying to stay comfortable.
I know what you mean about a water bed extracting heat from the body. I wonder if anyone else is already doing something like this or has discovered reasons why it isn''t effective or practical.