Dixie is correct and I have also seen them in the luggage areas at Target and Wal-Mart. You can also buy them online at magellans.com.
I personally prefer the flat "pouch" type wallets and not the hard plastic ones that are sold onboard many ships. The hinges on the plastic ones break easily and can be easily seen under clothing. They are far more apt to accidental opening and having your valuables end up on the ocean floor.
I also don't recommend keeping medication or anything in the wallet that will actually be damaged by contact with water. If not secured properly they will leak. Credit cards and cash work just as well wet as they do dry. :-)
Take care and have a great time.
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Possible solution: I wanted to take some photos at the beach with digital camera but did not want to get it ruined, and needed a place for valuables.
Here in Indiana we have a Galyans/Dick's sporting goods store (lots of outdoor camping,hunting type store like an Outdoor World.) I found perhaps not the most attractive but 100% waterproof hip sack. It was sort of rubber material that had flap you tucked over and then rolled top down and secured with the nylon parachute clip. I was able to swim and carry everything my family would need. It was roughly 6x8" if I recall less big rolled tight. They may have made fun of my ugly fashion accessory, but they came to me when they needed tylenol, camera, candy, and I even carried a wash cloth to wipe the salt water from our eyes.
If you do not want quite that elaborate, I saw very thick on a string ziplock type wallets. I totally agree with other posters, the hard ones do not hold up at all.
P.S. while not as popular as in the past, but sunglass leashes do come in handy while in the ocean, or taking a boat ride.
If I were you I'd put my camera in a regular zip-lock bag before you carry, or store it anywhere.. I've been doing that for years and never got mine wet although water did sometimes get into the carrying case I had it in.. Also, use zip-locks for your money and credit cards and tuck it inside your bathing suit, stays dry and you know where it is... Works great, try it... Bombero