I love this topic, and the varied answers.
I cannot believe how many posters are saying the OP had no right to say anything!
IMO, that kid was crying because he felt ashamed that not only had he peed in the pool, but that GRANDPA knew he'd lied about it. And from the rest of the story, it appears GRANDPA knew that wasn't right, and apologized for the boy. The boy lied, got caught, and cried over it. While you don't want shame and guilt to override your life, it is extremely useful in teaching situations and is probably one of the greatest motivators for learning that we have in our lives as children. How many times did you tell your toddler "not to bite Mommy, that hurts Mommy" when they were in that biting stage? You were trying to guilt them out of the behavior, and eventually (God willing) it worked. "You're not doing your best work when you get a C in math, Junior" - guilt and shame, what a combination. And a WINNING combination.
Here, the child was a little older than the toddler stage, and was engaging in behavior that needed to be corrected. The OP wasn't going to stand for urine in the "little pool" of the hot tub and had already been chased out of one pool, and the kid and his family clearly didn't care. He had every right to correct this boy, and it's sad that he had to - but he DID have to, because respect for others is one of the first lessons that should be learned, and one of the first things that we TRY to teach children. "Don't bite, share, don't pull hair, give that toy back" - but once kids reach an older age, some parents just stop teaching that simple respect. It's ok to send your kids off to school and have nothing to do with them for 7 hours a day - all the while, the teacher is correcting, instructing, and molding your children - but when another adult puts their foot down, it's not allowed?
Kudos, OP. I'd have done the same thing in your situation, and had the parents "freaked out" and went and got management

, I would have told them what happened and NOTHING would have happened to me from management. Maybe the kid will think twice about acceptable behavior in the future - or even twice about lying while Grandpa is around, which hurts no one!