Some of them were, ABCD, Chanties evolved from a wide variety of sources. We were talking here about how they started out. Some of them were rowing songs, stevedore songs, slave songs, minstrel songs, shore songs from the theatres, but most were probably started off by the chantymen themselves, and if we are to believe the few contemporary accounts the actual chantyman's lines were extemporised after the first couple of verses.

If you're referring back to the black watches who brought them onto the ships these particular ones were already being sung on the docks by the stevedores. Once the whole idea took off, pretty quickly I'd imagine, the basic outlines came in from all sorts of sources.

This is just an opinion but I'd imagine the simple short-haul and halyard chanties would easily have been made up by anyone aboard. The more complex capstan chanties would be more likely to have been based on existing shore songs. Amsterdam for instance is an adaptation of an English folk song. (not the one Hugill refers to).