Snakes can bite underwater, though they usually only do so if they're disturbed. Snakes hunt fish underwater and try to stay away from humans if possible.
The only venomous snake in United States is the cottonmouth, also known as the water moccasin. The best way to avoid cottonmouth snakes while boating is to not drift under tree branches, as cottonmouth snakes sometimes warm themselves in trees while waiting to attack prey below. If a cottonmouth sees the boat move, it may fall into the boat before realizing what is happening. If bitten by a snake, swimmers should seek medical attention immediately.