I've a vague recollection of hearing Stan Hugill say that the association of sailors with concertinas was largely a myth, and they weren't very common at sea, because they were relatively expensive, didn't stand up to marine conditions very well, and because of their shape were liable to roll about. However Dan Worrall's research seems to contradict this. Of course by the time Stan was at sea the concertina had already begun to decline in popularity.

In his Preface to Shanties of the Seven Seas he describes his father singing sea songs and shanties, accompanying himself on a button-accordion, so there is a historic precedent for using instrumental accompaniment, for entertainment if not in a working context.

Sailors at sea seem to have played every imaginable type of (portable) instrument. The fiddle was certainly prominent among them.