That's a Northern Water Snake, Nerodia sipedon. They are a harmless snake, but can be a little pissy if you pick them up. Pretty neat snakes really, they are a rattlesnake mimic and share a similar pattern and color to Massasauga Rattlesnakes. Their eyes are also on the top of their head so they can be partially submerged in the water with their eyes out of the water. They are often misidentified as "Water Moccasins" though in IL and the Great Lakes regions there are no Water Moccasins.