Barry, if I recollect correctly, Hugill's earliest citation of actual shantying - musical call and response, or line and chorus, as opposed to "sing outs" - was a description of a crew of black dockhands who came on board and used the capstan to load/unload the cargo.

But there's another factor to be considered. Was it Whall who observed that the old sailors sang in modal, while the younger sailors sang in major and minor, and the old salts were disgusted or resigned that the youngsters these days (turn of the 20th century?) just didn't have the ear to sing the songs the old way?

As Bronson points out, harmonization is a different matter in modal singing. It would have been a very different sound, and much sparser, than that to contemporary standards and rules of harmonization. One could easily audualize someone throwing in a fifth on "Haul on the Bowline", but that's very different from barbershop harmony on "Leave Her Johnny"....