1. Leaving a bunch of trash behind.
2. Letting your kids run wild. So other people feel like THEY have to watch them and make sure they don't drown.
3. Throwing sand.
4. Getting too drunk.
5. Playing your music too loud.
6. Encroaching on someone else's space. Like if you put your blanket down right next to them, or hit them with a Frisbee.
7. Being too loud in general.
8. Taking too many photos. Especially if strangers are in the background.
9. Fishing where people are swimming. So you have to worry about getting hooked.
10. MEN Staring at women in their bathing suits. You look like a creep if you get caught. So just do what most guys do . . . wear your sunglasses, so no one can tell.