Bah, cottonmouths have a bad reputation, but they're actually one of the more calm and predictable venomous snakes I've worked with. Water snakes on the other hand, are complete dicks and will crap on you and/or bite you if you get anywhere near them. :)

If there was a good rain, it could have been almost anything. Cottonmouths and water snakes both tend to head for higher ground when there's a good amount of water flow, but really, water snakes are -far- more common. Especially around areas where people frequent. Cottonmouths really don't like spending time around humans, whereas water snakes could care less.