The snake in the OP looks to me more like a either Midland, Northern or Brown Water Snake to me than it does a Cottonmouth.
It's hard to tell for sure without seeing the head.
The pattern on the snake looks 'broken' similar to water snakes whereas the pattern on a cottonmouth is more 'continuous' down the sides. I'm talking about vertically from the top of it's back towards the belly. Also, cottonmouths usually have a very thick body with a skinny tail at the very end.
Cottonmouth Photos I'm not a herpetologist and I may be completely wrong. I have known
this guy for most of my life and I've learned a little bit about native snakes over the years. I think non-venomous water snakes are constantly confused for actual cottonmouths. They do look real similar and it's hard to tell, especially if you only get a glimpse of them or their moving. Cottonmouths are mean, nasty, terrible snakes. They're very defensive to the point of seeming aggressive. I had a dog that was bitten 3 times by a cottonmouth once. Luckily the dog lived. I would kill a cottonmouth without hesitation in my yard.