Not having read Dan's books I hesitate in offering an opinion, but I would imagine cheap concertinas would have been as popular on board as any other cheap portable instrument, just as ashore.

Some more modern musicians however have found to their cost that taking a free-reed instrument onto the briny is not a good idea. They tend to rust the reeds very quickly with all the damp air. Be warned. expensive instruments have been ruined. (That's without taking into consideration all the pirates and sharks.)