If it's a watersnake (which it likely is), they are aggressive but not
poisonous. As to identification I can only recommend you search the net for
snakes common in your region.
We are repeatedly told by the SCDNR that there are no water mocassins in
Upstate SC but I am not convinced. Our backyard is a pond, and clearing the
brush and removing their hiding places will mitigate your problem greatly-
that and an active dog is your best option.