What state do you live in?
Could it be this: http://www.uga.edu/srelherp/snakes/elaobs.htm
If you have this snake contained, I suggest visiting your local veterinarian. Then you will know if the snake is poisonous.
It doesn't look like any rat snake I've ever seen, but it does bear an uncomfortable resemblance to a cottonmouth-a very poisonous snake that is comfortable in water, and has a very vicious bite:
I strongly suggest staying away from it, and changing the environment to drive it out if you own the land where you photographed it. The cottonmouth likes water, usually hangs in trees to drop down into the water, and usually strikes while swimming:
The triangular head is a sign of it being a venomous snake-please be careful around this thing and find out what it is as soon as possible.
Definitely a harmless Water Snake (genus Nerodia). I'm 100% on that. Probably a Northern Water Snake, Nerodia sipedon. It will bite if you pick it up but it is not venomous.
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