I saw one of these Snakes at, believe it or not, a Petland. It looked like an Eastern Garter, but it had a heavier head and had a greenish tinge to it. The closest thing I can come up with is a Buff Striped Keelback, but I doubt it...?
You might want to label all keelbacks as some form of HOT. All are part venomous, and their venoms have yet to be accounted for in toxicity. Be careful.
It might be a Xenelaphis sp. they fit your description fairly well. Often the pet trade names semi-aquatic harmless snakes as garters to instill some familiarity with customers.