We've been to the water parks many times.
We always arrive wearing our suits with a cover-up and sandals. We bring beach towels, beach bag, small cooler with drinks and sandwiches (no glass or alcohol allowed - they do check the coolers at the gate for these items), and a plastic box (w/ handle) to hold snacks, reading glasses, etc.
We are there when the park opens (you'll want to be there 15-20 min before announced opening) and then will head directly to a section and grap one chair for each member of our group (4). We spread out our towels to indicate the chairs are taken and then will tuck our bags, etc. either on top or next to the chairs.
Our sandals, shoes, camera, car keys, and other misc stuff has never been touched in all the times we've been there. We just keep these items tucked out of sight in our bags.
There have been years where our water park ticket was the same as our resort ID or was a PHP. These were definitely items that I did not want to lose. Those years, I would go back up and wait in line to get a small locker and then would store those items I couldn't afford to lose. They give you a locker key on a stretch band that you can just wear on your wrist.
Like I've said, though, we've never had a problem with our items being messed with at our chair location. I never take in more things (money) than I think I'll need either. All unnecessary charge cards, IDs, etc. I leave in the car or back in the room.
As a side note, the majority of the visitors at the water parks will be barefoot as they move around the park. You will see some with water shoes, but they are something that is totally optional.
Once you are settled, you may want to head directly for the slides as the lines can get quite long later in the day.
Have a great time!