My boat is a live-aboard, and I keep an Alvarez folk guitar onboard. I live in FL, so there is quite a bit of humidity, though I use a marine air conditioner which keeps the temperature and some of the humidity reasonable. I have noticed that the strings tarnish and need replacing more often than when I kept it off of the boat. The guitar seems to take the boat-life okay, but then again I would not consider myself a musician.
(I sort of make noise on it.