The cottonmouth (Agkistrodon piscivorus) is a semiaquatic snake of the genus Agkistrodon. These venomous reptiles also often are known by the name water moccasin.. Cottonmouths are large, hefty snakes that usually are deep brown or black in coloration.hey are prevalent especially in the southeastern region of the nation. The geographic scope of these reptiles includes Tennessee, South Carolina, North Carolina, Kentucky, Alabama, Texas, Virginia, Louisiana, Florida, Arkansas and Mississippi.