Northern water snake, Nerodia sepidon (non-poisonous). Color pattern is like a copperhead. Although you can distinguish based on the existence of a pit and the color of the back of their head, the practical way to determine they aren't the less-common copperhead is by behavior. Poke it with a stick; copperheads will become mad. Possibly saw several swimming across the river, although they're nearly impossible to identify while swimming.