According to A.L. Lloyd (Penguin Book of English Folk Songs), C.H. Firth (Naval Songs and Ballads, Navy Records Society, 1908) thought that the song's unusual metrical form might suggest that it had been -or was based on- an early shanty, but I have no idea if there's any evidence.  At any rate, it would probably be impossible to say what kind of shanty that may have been; though I suppose an expert might be able to make an educated guess, that's about all it could be.

Malcolm