Fort Zachary Taylor Beach is the best beach in Key West. (If you are looking for prestine, white sand, you don't want Key West...)  The beach is rocky, but beautiful.  Bring flip flops!  You can snorkel right off shore on several rock reefs, watch cruise ships enter and exit the harbor, snooze on the beach under the trees, or watch fisherman try their luck on the seawall... The beach has a reasonably priced snack bar, showers, and bathroom facilities.  It's worth the trip.