When I was a kid, my cousin and I were playing in the creek at my aunt's house and before I knew it, a big cottonmouth struck at me.
I'm not going to question your ability to ID a snake back then... but want to mention that cottonmouths don't strike very often. Their initial reaction is to just sit there... or, sometimes, slither away. If they feel threatened enough, they'll sit there with their mouths open trying to scare you.(where they get their name)
I've been VERY close to several cottonmouths and have never seen them strike(not to say they wouldn't)
Watersnakes on the other hand... if they feel even slightly cornered, they'll strike repeatedly.
Sadly, around here, EVERY snake seen at the water is labeled a cottonmouth... and a lot of water snakes get killed.
A few years back I found kids throwing rocks at something in the creek. I convinced them to stop. I went over, some guy was there and swore it was a cottonmouth because he saw a white mouth when it struck at him. Of course, so many of these people TRY to kill the animal which is most often the only time they're in danger... the person that is. When you go out of your way to interact(even to kill), you're putting yourself at risk.
This particular time, I went and captured the snake... it was a harmless water snake but many people there still wouldn't listen. :/