Well...the DLH has an awesome pool set up. But if you're talking off-property, I know the HoJo and Marriott both have water parks. I haven't personally stayed at either, though.
Hojo's has a great kiddie pool/splash area as well as a separate "regular" pool thats a really nice size.
Pic of my kids looking at the kiddie pool and also a pic of the regular pool.