I bought one of the waterproof camera bags last year from the waterproofstore website. It worked great! I used it on our cruise when we went to the beaches and for our Dunns Waterfall trip.
No sand to worry about, and great underwater shots. The only part I would warn you about is to watch out for water droplets on the outside of the bag before you take pictures. I took some pictures under water then above water and noticed when I viewed those pictures later that there were water droplets in the middle of the above water picutres.
I too had looked at the waterproof cases for my camera, but they are too expensive.. better off buying a waterproof camera.