That's definately a non-venomous water snake and it definately does not have vertical pupils. If you were able to get a close-up head shot, we could see that it has round pupils.
From the "Outdoor Alabama" website:
"Midland Water Snake - Common statewide, except southernmost portions of Coastal Plain. A conspicuous inhabitant of ponds, lakes, and streams, and the most frequently encountered water snake in the northern two-thirds of Alabama."
Baby Cottonmouths look very much like Copperheads. Here's a young Cottonmouth that I found in 2011:
It's not unusual for non-venomous water snakes to flatten their heads and either puff up or flatten out their bodies, making them look fatter, like this young Yellowbelly Water Snake I found is doing.