I saw this on another website & thought I would pass it along. This couple stayed in GrandCayman(not cruise ship).
If you go to the Public Beach portion of Seven Mile Beach there will be vendors there from the local hotel who will try to charge you $10 for a chair and a drink coupon. They are there specifically for the cruise passengers. If you tell them that you're not on a cruise, that you're staying locally, the price will mysteriously drop to $5 or even less.
Another man said that he vacations in GrandCayman every year and found that the prices go up when the cruise ships are in port. He also suggested to tell the vendors that you are staying on the island . . and avoid the cruise ship passengers.