Glow sticks in the pool, great for night swimming! Not sure this would be the best idea for a big in ground pool (would probably take a couple hundred sticks), but would definitely be cool in a kid pool or an above ground pool!
Fill the bottles with paint. Take a pin and poke a hole in the caps of the bottles. Get all your friends together Have a paint fight, like a water fight but with paint. (Wear white then take best friend pictures after, that would be so fun)