Not a dumb question at all. A dumb question would involve guessing at a dumb answer...

You've actually hit the nail almost smack on the head, according to my OED. According to them, Shanty derives (almost certainly) from "Chantez!" Sing!, the imperative of the French chanter, "to sing".

I would be prepared to make an educated guess that the reason for the survival of shanty in connection with the sea has a lot to do with it's survival as 1-2, 1-2 working song, the kind of simple repetative song/task that wold have died out much earlier on land in the face of increasing mechanization.