Along Plum Orchard Pond Trail behind the Visitor Center in St. Mark's NWR. I think it's a Yellowbelly Water Snake (Nerodia erythrogaster flavigaster), but not 100% sure, maybe a Banded Water Snake. The lower lateral yellow spots in the midsection make me question it a bit. This was a rather large snake, a least 4 ft maybe pushing 5 ft, which is large for either species, unless it's actually something else I haven't considered.

[UPDATED] It was pointed out to me on Flickr that this is a banded water snake.