We're huge fans of the water parks and have been to each one many times. We were just there in June and hit each one 3 or 4 times. The crowd never reaches the level that it does in the parks, so it's never THAT bad. But the most popular slides will get pretty long lines simply because they move people through slowly. The crowd will start to build about 60 minutes after arrival. If you get there at rope drop, you will absolutely get to do all of the big slides multiple times with little to no wait, so no worries there. Summit Plummet at BB builds up fastest because it's only one person at a time. Slush Gusher, which is right next door, will have a shorter line. Crush n Gusher at TL never really gets a monster line, it's usually under 15-20 minutes at most. I'd do Humunga Kowabunga before I did Crush.
You will have zero problems finding a chair at any time of the day, and a shaded chair if you get there within the first 30-45 minutes. There are so many chairs, they never all fill up. Plenty of shade to be found at both parks. If you're there at rope drop, you'll have your pick. Personally, I'd never pay the money to rent an umbrella and chairs, I can get them for free. At both parks, if all the shaded chairs towards the front of the park are taken, head toward the back. There are always entire sections of nearly empty chairs back there, with shade.
The area near Shark Reef has some things around to get shade. The reef itself isn't shaded, and the water is ice cold, but it's worth it.
I would also say that water shoes are a comfort/personal taste thing. I wouldn't even consider them for me. For me, there's something just really weird about wearing shoes in a pool, I can't do it. Yes, the pavement can get pretty hot at times, but there are ways to manage around it. Plus, I don't want to be bothered with having to hold them while I'm on a slide.