In the last 20 years there have been a number of hotels in the Los Angeles area that have had water slides built. If you ask your kids what they remember about their Los Angeles vacation, if it is a water park hotel, then they will often say the water slides.
Disneyland Resort Area
Disneyland does not have a waterpark, but there are an array of hotels including the Disneyland Hotel, that do offer water slides.
Disney's Grand Californian Hotel
They have a waterslide called the Redwood Creek Slide that is adjacent to large trees.
Cambria Hotel Anaheim
101 E Katella Ave, Anaheim, CA 92802
This Hotel has multiple slides.
The monorail slide structure is a must see.
Great Wolf Lodge
A large indoor water park in Anaheim's adjacent garden grove.
Hotel is across the street from California Adventure and has a small water slide.
Howard Johnsons Anaheim
Across the street from Disneyland with a water park and firework view.
Greater Orange County
Hilton Huntington Beach
Hyatt Huntington Beach
Hyatt Newport Beach
Rancho Pales Verdes