I too have some musical instuments. Three mandolins and two violins. Two of the mandos are acoustic, hollow body
wood, and one of the fiddles is acoustic.
I'm also a lifelong woodworker. And I think people paranoid about the effects of taking fine
instruments to sea are not looking at the issue correctly. The problems with humidity and wooden
instruments is not the humidity per se, the problems are from changes in the humidity.
Wood expands to various degrees when humidity increases, and it contracts when humidity decreases. When I was building furniture in
New England I had a lot more trouble with table tops and edge joined pieces than I ever had in
Houston. The difference is that the humidity on
Cape Cod goes from 90% in the summer to down less than 50% on some bitter cold high pressure weeks in the
winter. Woodworkers have to build things allowing for expansion and contraction. If you glue up a tight table top in the summer, it will open up with cracks in the
winter. OR if you move it to Arizona. Conversely, if you build it up tight in the dry winter in front of a fireplace, it will swell up and buckle in the humid summer months. And it does take some time to happen. The wood doesn't soak up or change moisture content overnight. You can wash your guitar down with a wet rag, no prob, right?
Now think about life at sea. You don't have these drastic humidity changes. You now live in a humid climate. I think in many regards it's actually better for some wood due to the relative stability of the humidity. It's not constantly expanding and contracting due to drastic long term seasonal humidity changes. And a week long rainstorm or dry spell isn't going to hurt it. You'd fly transatlantic from the
Mediterranean to Denver with it without a worry, right?
I've lived on Providenciales for ten years now, looking at the ocean. I've had stringed instruments I brought down from the US the entire time. My wife has an expensive KOA ukulele I bought her, made in
Hawaii, and they actually included a little humidifier bottle with it to KEEP it humidified.
The only issues I've had have been
steel strings rusting when not played for a while. E-strings especially like to pop and there's usually a rusty spot up between the nut and the tuners. I've found that going over the strings with something like olive
oil in the areas where you don't normally touch them seems to help.
So my vote is to not worry about the humidity. Yeah, keep the
salt water off it, but you're going to do that anyhow. You'll see less wood warpage on a
boat than you would driving cross country in an RV with it.
Hey, if you're worried, buy a cheaper guitar and watch that for a year to see if it hurts it. Once you know what to expect, bring your favorite ax on board.
I'm willing to bet there have been a few Stradivarius on some ocean liners for months at sea over the centuries. And they did not have the high tech varnishes of today. With absolutely no issues.