What's With All The Water Parks?

It seems that every respectable park has a water park inside it's gate these days, why? I didn't think that waterparks by themselves made much money, but including one in the park fee must really put a strain on finances.

When its 90 and above in the summer, people want to be able to legally wear swimsuits and splash people and ride slides, things thay can not be done in regular parks due to the dress codes and need for shoes on most rides anyway.
-------------
Six Flags America IS a Six Flags park!