Trust me I can tell the difference between a cottonmouth and a watersnake. The reason I made this mistake was because it was dark and I saw the pattern and then I just grabbed a box and he crawled right in. I never even looked at his head. as soon as I took the lid off, I immediately saw the head and instantly knew what I had.
By the way here's a pic of a watersnake I found one time. He was so nice I let him hang around my neck while I drove around looking for more snakes.