We have never been to TL and BB in the summer with the crowds. We were planning to arrive early in the AM before parks open. We have umbrellas/chairs reserved. I’ve been reading that crowds disperse around 4:00 PM and waterparks are much less crowded after that. Would we be better off waiting until late day to go there? Anyone have summer experience with this?
I have no info for you, as we’ve never been to water parks there at all, but since we’re going this July I’m interested too. Hopefully someone has experience with this and can help us out. 
Just like the theme parks, the waterparks take a while to get the crowds built up, and then things do thin out the last couple of hours, but I’d be cautious about waiting until late in the day to go. You know the famous afternoon rainstorms in Florida. It you limit yourself to a late day visit, the weather might thwart your plans!
The bright side of a rainstorm on a waterpark day is that, if you can wait it out, afterwards the place is deserted and you can get on every slide with no wait. One time we waited out a downpour under an awning with just my family and Goofy and his friend! Then went on all the slides over and over. My son thought it was the best day of our vacation.