Just to give you an idea, the water park capcity is about 11,000 people. Most days during the summer months the park closes to capacity at about noon and reopens at 2. If you get there at opening you can get the most popular rides done early. If staying on property utilize the EMH!!!
Rain is also a crowd factor. The summer months it is not unusual to have a afternoon rainstorm, if there is lightning within 5 miles they shut down the park. You don't have to leave, but you can't play in the water at all. This usually causes a mass exit and when the parks reopen it is less crowded.