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. A number of years ago, while trudging through the floodplain of the Congaree River below Columbia, SC, I had my most unusual encounter with a Cottonmouth. It was spring and the river bottoms held many depressions and drainageways filled with water. I was alone, which is not especially smart when you are miles from the nearest highway.