Well, Barry, I'm not sure from his description that we aren't saying much the same thing. It's interesting that he says that some people - presumably the better singers - did add harmony. However, harmony only works to a clean melody, and that's unlikely with the mix of voices and the circumstances - which is probably what he meant when he indicated it didn't come out like Sailor John and the Cathedral Quartet!

But your point about the West Indian and GSI singing is well taken. They had a tradition of complex rhythms and harmonies, and blacks from the West Indies, according to Hugill, were members of crews throughout the 19th century and probably earlier (the West Indies was a major 18th century destination), and by the latter half of the 19th century blacks from all ports made up most of the crews of whalers, for instance. I don't know how common they were in the merchant service, but it seems likely there was a lot of influence on singing styles as well as content.