Couple of things you can do; they make small waterproof containers to put everything in and just take it with you in the water. Problem is, if you're taking a camera with you - now what? They do make lockable containers that also have a wire that you can link through a chair or other object to protect your items. Pacsafe, kyssbags, and secure-it makes some of them, but there are plenty of other products out there.
If you're just taking a few items and don't have to worry about a camera, I simply use a ziplock bag and put it in the zippered pocket of my swimsuit.