A water moccasin is a venomous snake that lives in and around freshwater swamps, lakes, and slow-moving streams in the southeastern United States. Also called a cottonmouth (because it opens its mouth wide when threatened, showing white tissue that looks like cotton), this dark-colored snake can grow up to four feet long.

Water moccasins are strong swimmers and often rest on logs or tree branches hanging over the water. Unlike most snakes that flee when approached, water moccasins sometimes hold their ground and display that distinctive white mouth as a warning. Their venom is dangerous, making it important to give them plenty of space if you spot one near the water.

People sometimes confuse harmless water snakes with water moccasins, but true water moccasins have thick bodies, triangular heads, and a distinctive swimming style where their entire body floats on the water's surface, rather than just their head above the water.

The term water moccasin can also refer to a type of flexible, waterproof shoe originally designed for boating and wet conditions, though most people today simply call these boat shoes or deck shoes.