Key Lime Sailing Club, mile marker 99.3 on the bay side. It is not really a sailing club, but they have little cottages right by their beach. The owner, Paul Keever, has several catamarans of his own, from very little to very big, and they have some Catalina sailboats there that are free for guests to sail. Room rates are pretty reasonable, especially since they include use of a sailboat. Rates are usually lower down here starting about a week after Easter.
Key Lime is on Buttonwood Sound, which is a great body of sailing water and also gives you easy access to the "backcountry" of Florida Bay and some of the islands out there.
Only catch is that they have a minimum stay of a week (or maybe 5 nights). Check out their web site,
www.keylimesailingclub.com