West of Key West, the snorkeling at Marquesas Keys is known for its rich marine life and dense coral formations. But it is not known for great snorkeling since the visibility is usually limited.
The Marquesas Keys consist of 10 mangrove-covered islands encircling a wide lagoon formed from an ancient meteor crater. This is the only atoll formation in the Keys. The Marquesas Keys are uninhabited except for two private homes and the occasional tarp tent of a Cuban refugee. The islands were an old bombing range and there are even a few wrecks to explore.
Off this isolated archipelago, there are scenic reefs with diverse marine life to explore in waters from shallow to 20 feet deep. During Marquesas Key snorkeling, you may encounter eagle rays, loggerhead turtles, green sea turtles, lemon sharks and bull sharks. But, keep in mind that the waters are known for their strong currents and tidal surges.
The Marquesas Keys are located about 25 miles west of Key West, and they are protected within the greater Key West National Wildlife Refuge. To further protect the mangrove islands, it is prohibited to come within 300 feet of any shoreline. The only access to the Marquesas is by private boat or charter boat. In either case, you'll need an experienced captain familiar with the islands to find the good snorkeling dive sites. While there are some day charters heading out there mainly for fishing, there are also over-night, live-aboard dive boats making the run out to Marquesas.
It should be noted that it can be quite difficult to find a charter dive boat that regularly heads out to the Marquesas Keys, unless you plan to hire the entire boat which can be quite expensive. You'll need to start working the dive companies three or four months in advance and be very, very flexible with your dates. And, you'll need to reconfirm the trip weekly to make sure nothing has changed. And, even then, don't count on the trip going due to wind, waves, storms or other random acts.
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.
Overall, Marquesas Keys is the 14th most popular snorkel dive spot of all 40 snorkeling dives in the Florida Keys.