Since July is a very busy month, you're best bet is going to be to go on a weekday and early. On the weekends you're going to get some of the locals. I would honestly try to hit the beginning of the week as well. The biggest reason-- a lot of people start their trips on the weekend and as the previous poster said, the water park usually isn't the first or second thing on their list.
Seating and chairs are usually important to most (I know they are for me) and these can go really quick in the summer. So you'd really need to be there by opening. (preferably 20 minutes early). Also, the same thing applies here as the other parks. You can get so much more done in that first 90 minutes while the crowds are building.