I've said it was just a matter of time. And that time is now. The park-centric state of California will join the craze and get its first indoor water park. According to Great Wolf Resorts, the company has signed an agreement to bring a Great Wolf Lodge water park resort to Garden Grove, just minutes from Disneyland. At a planned 100,000 square feet, the indoor water park would be the largest in the chain's portfolio of North American resorts and one of the largest indoor water parks in the world. Plans also call for a 600-suite hotel, 30,000 square feet of meeting space, and other indoor entertainment features. The Great Wolf Lodge properties typically include a spa, restaurants, and attractions such as MagiQuest interactive games, miniature golf, and a children's club with a full range of activities. Pending approval, Great Wolf and its partners expect to break ground in 2011.

I’m not sure how well this will do. The water park will be restricted to hotel guests. Or so I read.
Hi Marko50-
Thanks for writing. That’s the policy for all Great Wolf Lodges, and the concept seems to work well for them. Their goal is to drive hotel occupancy, and the water park is the primary amenity to attract guests. My guess is that it will do quite well.
-Arthur