If you can live without a locker, it will allow you to get to things faster when you get to the water parks. We like to get there at opening, do the "must-do's" (Shark Reef at TL and Teamboat Springs at BB) a couple of times each before the crowds gather. If you don't have to stand in line for a locker, you can't do that.
I use a water-proof wallet to put my room key / charge card in and my car key. (We always drive to the water parks so we won't have to take Disney transportation while we're dripping dry!) Otherwise, there's not much else that I take that I couldn't stand to lose. (I take a disposable underwater camera with me, but it pretty much stays tied around my wrist. No one has ever taken it, though, even when I left it in my bag. I guess no one ever wanted to develop MY pictures.) But towels, water shoes, sunscreen, water bottles, etc. seem pretty safe.
Have a great time!!!
Julie