The third park is still being decided on, there have even been plans of forgoing a third park, and just building a fourth disney hotel, but we won't know until it is official. Right now the resort is working on our version of Tower of Terror, Flick's Fantasy, and re-vamps for Disneyland's 50th anniversary (flaming pirate skulls anyone?) We'll see what happens.
Right now, Soak City USA owned and operated by Cedar Faire, but under the title of Knott's is the closest water park to Disneyland (less than 15 minutes) and Travel channel rated it the 6th best water park in the country.
A little bit further away in San Dimas, we have the third rated water park in the country, in Raging Waters. It ranked behind Disney World's great water parks, and it has a great laid back So. Cal. feel.
Anywhoo, have fun no matter what you do.
Toodles
~Tina