Phil,
You appear to be jumping about from your wider references to 'chanty' itself. Please distinguish between the two otherwise you lose all of us.
Anything before 1800 has no relation to 'chanty' historically. The rowing songs of the Georgia Islands c1820 have so far not been related back to African rowing chants or any others in text or tune, whilst that is always a possibility. It is more likely that the rowing songs relate to river songs and field hollers from southern slaves. As far as I'm aware these are not African related in text but are almost always in English. The main impetus for shipboard chanties came from the Gulf Ports cotton screwers c1830.