They should have them at Target and maybe even at Eckerd Drugs. We saw them in Cozumel in a drug store. I took mine with me from home but found that I really didn't need them and took them off after a while.
MY SHIP HAS SAILED
Carnival Conquest 10/26/03
You can get both at WalMart too. I bought water shoes and waterproof container at Walmart two years ago and still use them. However, the shoes are not that good of quality and the inner sole liner tends to come loose after much walking, but for the $8 price what can you expect? They still work fine and they were cheap.
You can also buy these on board the ship, but you will pay a high price for them.
We live on the east coast and they have them year round at local stores near the beach. You could also pick them up in Florida if you have time before boarding. They sell the waterproof cases in the gift shop on board.