There's a couple of shower rooms located around the park, and then there's the main locker shower area right up front. I've never had an issue keeping my stuff in my bookbag while i'm playing in the park. I just head back to it every so often to check on it. I usually keep a huge mug of soda with me. So I go back and grab a drink. I just use my towel to toss over the bag and soda.
Plus I'm usually only in the roa river and walkabout waters. So I snag a chair in between.
I used to go to Aquatica several times a week. And always leave at park closing. So I would bring clean clothes, shower there, and everything else. I never bothered with doing that stuff after getting home. I would suggest bringing a bottle of swim shampoo, use that first to help get the chlorine out. Then you can use their shampoo, conditioner, and body wash. I love the smell of it. And of course because they have a water softener system, it was A LOT nicer than taking a shower at home. Only once, when it was pretty busy, did I get kind of cool water shower. Most the time, it was nice and warm without being too hot.