I can't say I've ever come across instances of the melodeon being used on a ship. Concertinas, yes; they were used on ships, but you can see that they'd be much more portable and easy to store than melodeons. You can certainly use a melodeon to much the same effect as a concertina for accompaying shanties, but I don't know if it was ever used much at sea. Sorry not to be more help.

Marje