Water Moccasin are not in the Upstate but info...

[River Rat]

Cottonmouths are large, aquatic, venomous snakes. They are generally dark above: olive, brown, or black. A lighter/darker cross banding pattern may be seen, especially on the sides. They are heavy bodied with keeled scales. Adult size is considered to be in the range of 30 to 48 inches. The young are more distinctly patterned, resembling a very dark Copperhead Snake, without the reddish tint.

Cottonmouths are common snakes found in wetlands, rivers, lakes, etc. Their range in South Carolina is usually limited to below the fall line. Many times non-venomous water snakes are mistaken for Cottonmouths. Cottonmouths get their name from their defensive habit of gaping their mouths open to expose the white lining of the mouth. This is their way of warning potential aggressors to stay away.

Cottonmouths can be aggressive. Unlike their Copperhead cousins, they will often stand their ground. Their venom is stronger and they tend to be larger snakes, making them very dangerous. I have encountered specimens which flee like water snakes, but more often they assume a defensive posture or crawl/swim slowly away.

Resource
www.snakesandfrogs.com/scra/snakes/cotton.htm