P3
The two principal Gulf cotton ports from 1830 to 60 were New Orleans and Mobile, both settled by the French; both still celebrate Mardi gras. I have not found any accounts of loading at New Orlaeans in this period, but there are two of Mobile that illustrate clearly both the source of the term and the circumstances of its transfer to a working song afloat. the first is from 'Twenty Years at sea, by F S Hill (1893), describing a visit to Mobile in 1844;

'the cotton....was now to be forced into the ship, in the process of stowing by the stevedores by very powerful jackscrews, each operated by a gang of four men, one of them the 'shantier', as he was called, from the French word 'chanteur', a vocalist. this man's sole duty was to lead in the rude songs, largely improvised, to the music of which his companions screwed the bales into their places....
'A really good shantier received larger pay than the other men in the gang, although his work was much less laborious. their songs which always had a lively refrain or chorus, were largely what are now called topical, and often not particularly chaste.'