There isn't really much in the way of beaches in Key West. We always go to Baha'i Honda for a day when we visit.

The smallish beach near the Southernmost Point may be the closest to what you are looking for but it (as well as others) may be owned by the B & B there...where are you staying ?

We have rented a house in Old town

Most of the water sports can be found @ 0 Duval St. From rentals to Charters, to glass bottom boat rides or even the Conch Train (which is money well spent if you have never been to Key West). Also take a look @ the post about the husband REALLY wanting to go to EYW, you should find it helpful.

There are beaches along Atlantic Blvd, Smathers beach being the biggest and these are on the east side of the island. They have vendors who offer water activities.

In the historic area there is Fort Zachary Tayor. Look for signs off of Whitehead Street around the Truman Annex. (They are brown park signs) The beaches there are the best (imo) in Key West. There are rocks in the water that you can snorkel around and see fish.

As mentioned in other posts, Bahia Honda State Park has nice beaches. It is 37.5 miles north of Key West. When our kids were younger we always camped there.

I had forgotten about the beach over by the Fort, that would be my pick, on island.

FWIW there is one alternative to a beach in Key West; Dante's, a bar and restaurant just past the Half Shell Raw bar, that has a pool and tiki bar that you can use as long as you buy something - a drink or a snack. The food is not bad there and they have a good raw bar, and have started having live entertainment some afternoons. The pool is not very deep, though.

http://pjk.smugmug.com/photos/159448628_rgmLX-M.jpg
http://pjk.smugmug.com/photos/159448634_un7T8-M.jpg

Pjk

Here is a link to a map and a short summary of some of the beaches in Key West. I hope it helps, have a great time.

http://www.keywest.com/beaches.htm

The beach at Ft. Zachary Taylor is the nicest in Key West, but you'd need water shoes as it's rocky. I might add that it's man-made, but so are 99% of the other beaches in the keys.