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I'll vote for water snake, as well. Several reasons - mostly because they are only extremely rare visitors to Indiana - even in the southern most counties. Now it could be that one of our "south of 40" members hauled one up (more likely south of I-64).  Regardless, watersnakes are mean sunny-beaches and don't like to be petted. So leave it alone.  If it is a cottonmouth, it'll freeze to death over the winter.  

Where I live in the Pidgeon Creek bottoms there is another water snake that lives in my backyard at times and in the swampy areas around me. It is like the banded snake, non poisonous. It’s called the Copper Belly water snake. These are spooky because they exhibit some of the traits of a cotton mouth. Short stout body length, similar color pattern and a sort of diamond shaped head. They can be an aggressive snake also. They are protected by law. Still to a startled outdoors person and at first glance you will think this is a cottonmouth and it’s time to be somewhere else.