A few months ago I was at a water park with friends, and after having fun on the slides etc we found some chairs and relaxed for a while and then decided to have a quick cool off in the pool in front of us. When I returned to my chair there was a young girl of about 12 sitting on my towel, and she was dripping wet!!! I was so annoyed. She had also moved my hat and sunscreen.
It wouldn't have been so bad had she bothered to move my towel, but I was very annoyed as I had to hire the towel and now it was soaking wet and I had nothing to dry myself with. The thought of using the towel now it was all wet after she'd been on it sort of creeped me out a bit too (but maybe I'm over sensitive). I had to ask her specifically "could I have my seat back please, you're sitting on my towel", before she would even move.
The odd thing was that there was an empty seat next to the adult she was with, she could have sat there.
I think it's probably not so bad to move towels out the way, if they are dry and the seats have been unoccupied for a while, provided that you are willing to move if people come back and ask you to.