There may be a water slide at a hotel pool in Los Angeles proper, but we have yet to find it. Terranea is just outside the city limits, in Palos Verdes, and while the prices are as steep as the cliffs overlooking the Pacific ocean, the pool is made for kids and fun. We reviewed it here.
The Peninsula Plunge is a 140-foot waterslide that leads into a heated splash pool. It’s a pretty big pool area with a fountain and a separate adults-only area also. They keep the water nice and warm, so it would make for a great in-town vacation anytime of year.