I don't liveaboard
, so take this as you may, but I have a Takamine guitar that spends weekends on the boat when we're just drinking beer
and hanging out. I definitely notice tuning changes from the time I put it in the case at home versus when I take it out of the case on the boat. However, it has an onboard tuner, which is incredibly handy. It also has a steel
rod in the neck to restrict the warping of the neck and allow adjustment.
I went with the Takamine over a similar Martin because it was solid wood whereas as the Martin, which was more expensive than the Tak but the cheapest Martin in the store, had a veneer top.
I've had the Takamine four years now, gigged it, camped with it, boated with it - no problems at all.
I'd say with any stringed instrument that's going to be in harsh environments, go with solid wood and look for a metal truss rod in the neck.
It also probably wouldn't hurt to keep a couple packets of dessicant in the case and in the body of the instrument itself.