How about a harp?
Let me assure you, if you think a guitar is hard to keep in tune aboard...
Whenever I take off on an extended solo cruise, I bring a little folk harp I built from a kit. The kit was relatively cheap, yet once completed it had a better feel and sound than many higher-end production units (I like woody rather than tinny harps.) Even the little folk harp is too big aboard my little boat unless I'm soloing, though.
This particular design uses ply for sounding, and heavy solid cherry for the box, neck, and column. While the humidity is unlikely to directly harm the instrument, for the first week or so aboard it is plainly cantankerous to keep in any sense of tune. Tuning sessions take up a half hour, at least, almost as bad as when I'm stretching in a new suit of strings.
Fortunately for everyone in the anchorages I drop into I'm both a poor player and (I hope) a considerate one; I practice below, and not at all if anyone is hooked close by.
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