There is no difference between a water moccasin and a cottonmouth. "Cottonmouth" and "water moccasin" are two different common names that are applied to the same species of snake. Known to scientists as Agkistrodon piscivorus, cottonmouths are semi-aquatic, venomous predators that are native to the southern and eastern portions of the United States.
Called "water moccasins" in reference to their fondness for aquatic habitats, the snakes earned the name "cottonmouth" from their peculiar defensive display. When confronted with a predator or threat, a cottonmouth coils its body and opens its jaws wide, displaying the stark white interior of its mouth. During this display, cottonmouths may also vibrate their tails and emit musk.
Cottonmouths are generalist predators that consume a wide variety of species. They hunt for fish, frogs, rodents and other snakes - even other cottonmouths. Cottonmouth venom helps to digest the tissues of its prey, which accelerates the rate of digestion.
Cottonmouths give birth to litters of live young. When first born, the young have bright yellow tail tips. The young snakes use these yellow tails as lures; by wiggling their tails in a manner reminiscent of a worm or caterpillar, they attract frogs or lizards within striking distance.