Help identifying snake?

I'm on a NW MIssouri farm and keep seeing this snake in my pond. I will pay more attention to the head shape next time I see it. It is a medium length, maybe 2.5 to 3 feet, but stocky-bodied, black with brown hour-glass shaped markings. From pictures I have looked up, it is not a copperhead or water moccasin, unless there is a variety that is black bodied with lighter markings. The markings are a horizontal stripe on the back that widens to a diamond shape on the sides.