On the south side of Marathon Key, the snorkeling off Sombrero Beach is popular with local families. Within the designated swimming area, there are calm sea grass beds among the sand flats that are habitat for starfish, lobster and crabs. Plus, from April to October, the beach area is known for its hawksbill turtles. However, the water clarity is usually pretty poor unless it is super calm and windless for several days in a row. Even with clarity, there is usually not much to see. The best bet is to try snorkeling along the rocks at both ends of the beach during high tide. While snorkeling in the Sombrero Beach swimming area, a diver down flag is not required. You may want to snorkel here at high tide for best visibility.
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 Beach 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. Keep an eye out for the Portuguese Man-O-War which frequent this beach.
To reach the snorkeling at Sombrero Beach by car, take US 1 Overseas Highway to the SR931 turnoff at mile post 50. Head south on SR931 (aka Sombrero Beach Road) for 2 miles to the end where there is plenty of parking.
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 Beach is the 4th most popular snorkel dive spot of all 40 snorkeling dives in the Florida Keys.