I'm a big fan of sitting around the cockpit and playing music with friends. Some of my favorite cruises have revolved around music.
As for the effects of a water environment on guitars (and I think this echoes what some others have said), a friend of mine used to bring his Martin D-35 with him all the time on board for weekends and extended cruises on his boat. That guitar now has a split on the side. I have a Martin HD-28, and because of his experience, I refrain from bringing it sailing (or anywhere like that even). I either bring my Backpacker, or if I want a full-size, a old Alvarez or Washburn, neither of which I worry about (both are veneer, not solid).
Anyway, have fun playing on the water!