More descriptions from the decades before I allege the "new chanties" came in.

The first reference is not very insightful re: chanties, however, such early references to African-Americans singing during work are hard to locate, so I give it FWIW. It comes from "Abridgment of the Minutes of the Evidence Taken Before a Committee of the Whole House," including depositions from 1789. In one passage, the following bits of info are given with respect to African slaves working sugar cane fields in Grenada.

The cutting of canes is not very hard, tying them easy; the feeding the mills and sires are the most laborious. The rest of the work is very easy. On the whole, thinks the negroes are most healthy in, and like the crop best. Never knew them complain of work then. The mill- gang commonly sing all night. Certainly labour in crop is the hardest, as 1/2 their time, out of crop, is weeding. Holing is the most severe work out of crop.
...They often give holers weak grog twice a-day. Holing does not occasion sickness. Negroes seem fox'd of it, and commonly sing at at.

Of course, the assumption that the slaves were having fun --more so because they were singing-- was probably misguided. (Many years later, Frederick Douglas attempted to correct this assumption, arguing that the singing was not to express joy but rather to overcome sorrow.)

Next reference is to another rowing song reference. It is interesting for what seems to have been a transitional part-English/part-African dialect singing, and for the spread of this practice throughout the African Diaspora.

NOTES ON THE WEST INDIES... VOL. 3, by George Pinckard, pub. 1806.

The author's observations appear to be from the English West Indies Expedition of the 1790s.   He is traveling by boat down the Demerara River towards Georgetown (then known as Stabroek). Black slaves are manning the oars. Here is the passage, pg. 322:

Observing that they rowed with languor, and that we made but little progress, the cockswain was desired to exchange the helm for an oar, and to enliven his comrades with a song, encouraging them to join in chorus, and to pull together in musical time. This operated with magic effect. Every slave was inspired, and forgetting all sense of fatigue, they again pulled with unwearied vigour. We were not more pleased with the result of the expedient, than amused by ihe ready ingenuity with which our wizard cockswain composed his appropriate song, and gave it all the effect of enchantment. Resigning the helm to the weakest slave, he placed himself amidst the crew in the centre of the boat, and pulling his oar stronger than the others, he invented extempore lines for a favorite African tune, finishing each stanza with "gnyaam gnyaam row" "gnyaam gynaam row" in which all were to join by way of chorus; and we found that " gnyaam gnyaam row," never failed to give additional force to the oar—and consequent head-way to the boat.

The names of the slaves, their wives, their food, drink, and all their pleasures were introduced in song, and tuned to the pulling of the oar : likewise the names of each of the party whom they were rowing, their professions, qualities, and occupations, and their several intentions towards the crew, all made a part of this inspiring air, which, however ridiculous in the words and music—in its effect succeeded even to a wonder. The pulling of the oar, the directing of the helm, even the position of the slaves in the boat, and the compensation each might expect as the reward of his exertions were all adroitly included, and "gnyaam gnyaam row" accompanied each stretch of the oar in chorus. Led on by these persuasive themes, each seemed to emulate the exertions of the all-animating cockswain, and, throwing off the heavy marks of fatigue, they conducted us merrily and speedily to "Garden-Eden."