11am - 4pm is the majority of the time the water parks are open, and yes, it's quite crowded then during July. You can't get all of the rides in the water park done between 10 and 11, no matter how fast you are. I'll also assume by the tone of your post that you've never been at a water park on the same day that a large Latin American tour group shows up...They really add to lines and wait times.
Also, it often rains during the mid and late afternoon in summer. A thunderstorm can lower crowds, but it also closes rides, sometimes permanently. I have been there on some days where the rain and/or lightning sighting is so persistent, the park's slides are mostly closed from mid-afternoon onward, so getting on slides after 4pm isn't always an option in summer.
Sure it's nice to have short lines after a thunderstorm (after waiting under shelter for 1-2 hours for the storm to end), but it stinks when after 20 minutes, the slides are closed again because more lightning was spotted. And then it starts to rain again...
Nonlocals who only have one day devoted to the water park have no choice but to do the slides midday, because that might be the only time available to them.