Towels can be rented ($2, I think) but they are not provided for free.
We tend to use the early morning "rope drop" water park strategy, arriving before the crowds, which we've seen grow around lunchtime, as people have hit the theme parks for a few hours and are ready to chill out in the water.
Based on PP's successful approach of waiting until mid-afternoon to hit the water parks, it sounds like there is a peak crowd time of probably 11:00 to 3:00...and early afternoon is also when there is likely to be the typical Florida brief but strong rain, which can shut down a water park temporarily if there is lightning. Assuming you're not intent on staying all day, I'd focus on one extreme (early morning or late afternoon) and extend your time as you see fit.
We have found that hitting it first thing in the morning, we can ride pretty much everything at least once without any crazy wait times. Then if you want to hang around with the crowds, you can do so in the wave pool, on your lounge chair or eating a macaroni and cheese dog from AvaLunch!