Too, after completion of the Erie Canal, one needn't rely only on coastwise traffic. There was the option of transport west to the Allegheny River, and down the Ohio. Water almost all the way -- fast and cheap.
But the logic falls apart in light of the claims that the south paid anywhere between 80 and 90 percent of the tariffs. If that percentage of the imports was destined for southern consumers then why stop in New York to sort things out when just about all your cargo is going to the south to begin with? Would it not make more sense to go to Charleston to sort things out and then tranship the small amount destined for the North from there? And why would intercontinental ships want to resupply in New York if the were going to pick up their cotton in New Orleans or Mobile?
I just finished reading William Least Heat Moon's account of his boat trip across America (River Horse) which featured that old route.