Wildlife Spotting

Unfortunately Jonathon, the only way you can really know what is a venomous snake and what is not, is knowing what the venomous snakes are and how to recognize them (which is much easier than knowing all the nonvenomous). There's no cheat sheet for the average person. For example, if there are copperheads in the area, know how to recognize the copperhead. If you find a snake that doesn't fit that description, while you may not know what it is exactly, you at least know it's not a copperhead. It becomes a little more difficult when you have species that look similar (e.g. Agkistrodon piscivorus vs. Nerodia sp.) or behavior mimics but what I said still applies.

Ok, well you just ruled out every identification technique that I have ever heard of. I do know that none of those things is completely accurate, but when all of them combine, it gives a pretty good estimate. So how do you suggest identifying snakes such as this one?

Your suggestion is correct about this being a Northern Water Snake, but everything you just said to rule out being a venomous snake is not correct. No snake always swims on top of water or under water, vertical pupils means nothing, and almost every species can make its head look wide and angular shaped.

Since it is swimming in the water rather than on top (like the venomous snakes do) and since the eyes do not have vertical pupils, and since the head is not wide and angular shaped, I believe it is a harmless water snake. Given that it was in Ohio, I think the Northern Water snake is the correct species.

Maybe a banded water snake

That looks like a copperhead to me.