I've had guitars on boats for many years.
My old $100 Yamaha has held up well.
I once owned an Ovation because it had a fiberglass
body and I thought it would be the one to have, but it has a spruce top and it warped and it always sounded like a garbage can with strings anyway.
I just bought a Voyage Air and I love it - so far, so good and not a lot of $$$.
I picked up a Martin backpacker and got rid of it - no sound at all, never enjoyed it at all.
You can't bet an old Yamaha fg180 red label, if you can find one for around cheap
When they built these guitars, they varnished the insides as well as the out and sealed them up pretty good. I've had mine since 1969 and it was worn out when I got it. It has been on my fishing
boat with me from Mexico
. It has lived in the dessert, mountains, on the coast on several boats and I've never done a damn thing to it and don't really take care of it like one should. It's starting to look like Willie Nelson's guitar as I've almost worn a hole in it, the frets are flat and it is as worn as any guitar ive seen but it plays and sounds as good as any and better than most, even a $3000 Martin doesn't sound any better or play any better. I recently bought a 500l9 Taylor short scale and it has a better one but doesn't play any better and cost me a bunch.
I'd go for another Yamaha, but an old one if you're looking to go cheap