This was either a water snake (Nerodia sipedon) or a copperhead (Agkistrodon contortrix), I couldn't get close enough to see the banding clearly. I don't know if it's possible to tell by the fact that it was swimming with it's back at the surface, or by the shape of its head (considering the poor photos). It completely submerged at one point as well, to swim under some rocks. I'd guess between 24 to 36 inches long.