As a guess I'd say it's probably the nearest thing we have in English folk song to an example of communal composition, i.e., someone thought of the basic idea and others chipped in with verses

Isn't that the way with all folk songs? The essence of the Folk Process being a creative one as these things get passed around and customised accordingly.

I got Candlelight Fisherman from the Topic Bob Roberts album (Songs from the Sailing Barges) although when I played it to my mother once she said it was one of the songs my late father used to sing (Bell Bottomed Trousers being another). I've no idea how he came by them though I guess his love of all things maritime had something to do with it. I've got a different tune for it which I can't readily account for.