Are you sure they reopen if enough people have left the parks? Down here in another warm part of the world (Texas), we've seen people turned down late in the day at water parks, where the park has been crowded earlier but is no longer crowded. They say it is due to the chemicals and water quality...it's probably governed by state or local law. I guess the chemicals can keep water clean for a certain number of bodies per day. Around here, when they reach that limit, no new people get in, even when the crowds leave later in the day...but if you've been in, had your hand stamped and then come back, you can get in because you've already been counted.
Does that make any sense?
I'd go early rather than late if I were you. When you think that late in the day the chemicals may not be killing all the bacteria and other yucky stuff left in the water by all those thousands of people...doesn't sound so desirable, does it?