Snakes can open their mouths and bite if provoked underwater.
Although the water moccasin, or cottonmouth snake, prefers to lounge on logs or tree limbs at water's edge, it has been known to capture its food while swimming underwater. By the way, rattlesnakes have the ability to swim and bite underwater as well. For more information check out the Cottonmouth Water Moccasin Exhibit at Bayou Bob's Brazos River Rattlesnake Ranch online.