Southwest of Key West, the snorkeling off Sand Key is popular for its extensive staghorn coral and fire coral in waters with good visibility. The Sand Key is only exposed at low tide, but it is marked by a lighthouse and mooring buoys. With water depths ranging from surface down to 65 feet, you can expect to see marine life including seargent majors, midnight parrotfish, french angelfish, queen angelfish, hogfish, wrasee, damselfish, barracuda, nurse sharks, green sea turtles, southern stingrays and the non-stinging cone jellyfish. This is a good snorkeling dive site that is known for usually good visibility. The Sand Key snorkeling is quite popular with Key West charter tour boats, so it can get crowded at times.
Another good nearby snorkeling dive is Rock Key where there are two shallow wrecks to explore. Keep aware that the waters around Sand Key and Rock Key may have surges.
To reach the Sand Key Sancturary Preservation Area snorkeling from Key West, you'll need a private charter boat for the half hour excursion. Sand Key is located about 7 miles southwest of Key West and 1 nautical mile west of Rock Key. A good private boat launch is the Key West Garrison Bight Marina. Out at Sand Key, there are 24 mooring buoys. The southwestmost buoys are the best in most conditions, unless the wind is from the west when the eastern buoys are better.
To reach the snorkeling in Key West, fly into Key West International Airport (EYW), which is served by several airlines including United/Silver Airways, Delta, USAir and Southwest. Alternately, there is also the regular Key West Express ferry service (4 hours) that runs from Ft Myers.
When using a dive boat charter to reach the Sand Key dive site, make sure to reconfirm the reservation the afternoon prior AND the morning prior. Key West snorkeling tour operators are notorious for changing the dive destination just before pulling out of port. Actually, it's kind of comical getting them to commit to which dive site the boat is actually going until you are out of port. The reservation agents will usually treat you as very ignorant and just say they are going to the reef-off-shore, like that's an actual dive site! It is very difficult to pre-reserve a tour to a specific snorkeling dive site. Luckily, the tour companies are all located near each other in the historic Key West wharf and you can just walk-up 45 minutes prior to depature and ask where they are going. Also, keep in mind that most dive boats have a minimum passenger requirement in order to leave dock meaning they may cancel at the last minute, so when reverifying the afternoon prior ask how many reservations they have. And, they may cancel due to wind, so call an hour prior if in doubt.
Overall, Sand Key is the 23rd most popular snorkel dive spot of all 40 snorkeling dives in the Florida Keys.