Raging Waters in San Dimas is about 45 minutes away. Six Flags Hurricane Harbor is about an hour. There aren't a lot of water parks around here, the beach is close and free.
The closest is Soak City at Knotts Berry Farm. However, if you are planning on spending any significant time, I recommend Raging Waters. It is more inland, so you'll get warmer weather, and the park is large with a large variety of slides, pools, and lazy rivers to float down.