I've always thought one of these would be dandy for the boat. It's an 1840's melodeon. The legs fold up. The pedal provides the power. One upside is that it will never go out of tune. One downside is that it does not have a case. A second is that it's made with hide glue which is sensitive to damp. But the most important downside is that there is absolutely no connection between the music notated on the page, my eyes, my brain, and my fingers.
(We've got two of these little folding reed organs. I'm finally, after 30 years, finishing the restoration on the one pictured, making the missing swell pedal and associated parts. The second is a total basket case but I now have the repair materials in hand, and I've started. Entertainingly (to some), I bought a third, larger melodeon last weekend. Could not resist the price ($36.(!)) and it has a lovely rosewood case, removable legs, almost works, but lacks its pedals.)