Hi REZ4JC! I lived in the Keys a number of years back...and the number one misconception of the Keys is that there are beacches. There is one public beach in key West called Smathers beach and it was "okay". Since the Keys are surrounded by coral reef they have to truck in the sand to make a beach. There was a really nice beach at the Marriot on Reyonlds Street and I believe there are a few beaches at the big hotels near the pier where the sunset celebration is. maybe someone else can help withthe names? I don't know if the hotels allow non guests to use the beaches though.
If you are there for the day, I would try an excursion to go snorkeling. You would have a great time.