An educational trail sign says the rabbits in the swamp are Marsh Rabbits. This is as good a look as I’ve gotten and I’ve read that the Marsh Rabbit is a strong swimmer. He’s on a small island in the flooded marsh so it seems to fit.
The other possibility would be an Eastern Cottontail, but he was not moving to show his tail while I was watching.
He was well camouflaged among the Cypress Tree knees.
Audubon Swamp at Magnolia Gardens, Charleston, SC.