In preparation for our caribbean cruise and snorkeling we purchased a couple of waterproof "Splash Caddy" brand waist packs to hold critical documents while snorkeling on our person. We ordered them on-line at Magellans.
They worked down to 20' and kept passport, cruise card, cash and credit card completely dry. They come with a spare liner so don't buy extras. We tried the large waist pack, the small waist pack and the round the neck pack. I would not recommend putting keys or sharp/bulky objects in them, only flat smooth objects. This helps protect the seal. While snorkeling, the little amount of air left in the "ziplock" type bag, but of special design and thickness, floats the waist pack, so you don't have to worry about dropping it.
These would be great for the beach and splashing around. However, while snorkeling on a line sponsored excursion, our valuables would have probably been safe left in the daypack on board.
So the bottom line is if you want it to stay dry and on your person, they work.