He's a banded watersnake, a common variety that is often mistaken for the poisonous cottonmouth.
Funny you should mention that.
When I first saw your photo I thought "HOLY JUMPIN' CHRISTMAS! DOES INDIANA REALIZE HE'S HOLDING A COTTONMOUTH -
BY THE TAIL?!?"
Word to the wise: it's best not to hold a poisonous snake if that situation can be avoided, and most particularly not by the tail.