All the points I was going to make in response to the original poster have been thoroughly and eloquently answered.
One thing, however, that I continually have a pick about is the assumption that just because you're on a boat your instruments will instantly corrode and rot into a puddle of goo, which in my experience has simply not been the case. Granted, I also have a bad habit of taking my instruments (a Morse Geordie in the concertina dept. and a fiddle in the "other" category) camping, but never mind that. I can't speak for ocean sailing, because I've not done it yet, but during my time on the Great Lakes I never had a problem with my instruments being damaged due to excessive humidity, because even though you're on the water the ship acts pretty much like a decently protective building. Common sense applies too, clearly if the whole watch has just come below in their dipping wet foulies and the berth deck is a dank mess, it's an inopportune moment to grab your box and play a few tunes. But for the majority of the time a good case and storage in your bunk (or if you're in hammocks, the sail crib) is a perfectly acceptable environment for an instrument.
Anyway, it's always good to see another sailor on here.