South of Marathon Key, the Sombrero Reef is by far the most popular snorkeling dive site in the Middle Keys. There is a large spur and groove coral reef with star coral, brain coral and elkhorn coral. The snorkeling depth at Sombrero Reef ranges from 2 to 25 feet deep. There are some nice grassbeds that are perfect for beginner snorkelers to explore.
The Sombrero Reef snorkeling is located about 4.5 miles south of Marathon and the reef is marked by a 142 foot lighthouse. There are several 11 buoys. If you are accessing via a private boat, use the 2-Mile Bridge boat ramp. Just keep in mind that the place gets very crowded on the weekends. Although this is a protected Sanctuary Preservation Area, watch out for trolling fishermen which are permitted here. Also, the area is known to have strong currents.
Tip - After a long day of snorkeling at Sombrero Reef, then grab some grub conch at Morada Bay Beach Café in Islamorada or Saltys Waterfront Grill for some cold drinks. And, if you didn't happen to spot any turtles while snorkeling off Marathon Key, you can always stop by the Turtle Hospital for a top of the hour tour of the hospital or turtles tanks located at mile post 48.5.
While the snorkeling at Sombrero Reef is possible year-round, most snorkelers usually shoot for March through September. There is usually some rolling surf at most snorkeling sites off Islamorada, while you can expect the flatest surf conditions from June through August.
When using a dive boat charter to reach the dive site, make sure to reconfirm the reservation the afternoon prior. Most dive boats select the site the afternoon prior or even the morning of the dive. Also, 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. To reach the snorkeling in the Florida Keys by air, there are several options. The most popular option is to fly into Miami International Airport (MIA), which is served by all major US and international airlines. Also nearby, a potentially lower cost option is to fly into Ft Lauderdale airport (FLL). From either of these airports, you can rent a car to drive south on the Florida Turnpike a couple hours to reach the Florida Keys (watch out for automated cash-less toll highways along the way). Alternately, you can fly into Key West International Airport (EYW) which is served by several airlines including United/Silver Airways, Delta, USAir and Southwest. If you don’t mind coming into Key West, there is also the regular Key West Express ferry service (4 hours) that runs from Ft Myers.
Overall, Sombrero Reef is the 5th most popular snorkel dive spot of all 40 snorkeling dives in the Florida Keys.