As a non-English native I didn't understand: Were there alligators in the river? Were the kids afraid of their own imagination? Is there a double meaning to the word "gator"? Thank you
There _were_ alligators in the river. He almost certainly imagined the gator next to him in the water, but swimming in it was still very dangerous. Gator has only one meaning.
The protective cloud of pee is just a joke. It's funny because pee is useless to protect you from alligators. It's also funny because peeing yourself is funny.
There were definitely alligators. I doubt the author was trying to say they were afraid of their own imaginations. It's possible that he imagined the alligator that was next to him in the water, but it's equally possible that he wasn't.
