Key West is far from a "beach town", my personal opinion of the best beach is actually closest to the cruise piers, it is Fort Zachery Taylor State Park. Depending on what pier your ship is at you can actually walk to it, admission is the best bargin in Key West for something like $1.50.
Fort Zach - <http://www.floridastateparks.org/forttaylor/default.cfm>
Here are some of my pics from a recent trip to Key West, Fort Zack is in the middle of the pack:
<http://www.wiscomm.com/EYW>
Here is some additional information on the Key West beaches:
FORT ZACHARY TAYLOR
An 87 acre state park with a pre-Civil War Fort. Wonderful sunset views. One of the best beaches in Key West, though a bit rocky. Public restrooms, food/drink kiosk, chair rentals, picnic tables and BBQ's. Admission charged. Entrance through Truman Annex at Southard Street.
SOUTH BEACH
A favorite of locals. Nice sandy beach with shallow waters and a concrete pier. No restrooms or facilities. Located at the end of Duval Street on the Atlantic Ocean.
DOG BEACH
A canine favorite though rocky and slippery. No restrooms or facilities. Located next to Louie's Backyard (Vernon and Waddell)
HIGG'S BEACH
Wide sandy beach with long wooden pier. Restroom facilities, covered picnic tables, full service restaurant, playground, chair rentals, rafts and watersports rentals. End of Reynold's Street.
REST BEACH
Soft sand and natural vegetation. Home of the White Street Pier where you can enjoy some great fishing. Wheelchair accessible. Yoga deck with nightly classes. Restrooms within walking distance. Picnic tables. White Street on Atlantic Ocean.
SMATHERS BEACH
Half mile stretch of sandy man-made beach. Restrooms, ample parking, concession stands, chair rentals, bike paths, picnic tables, volleyball nets, watersports rentals - jet skiiing, parasailing, and windsurfing. South Roosevelt Boulevard.
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Please let me know if you have further questions, I would be happy to help with the America's Greatest Island!
Craig
MKE/EYW