On Key West, there is good snorkeling to be found at the Key West Marine Park. The marine park is maintained by the Reef Relief non-profit organization based in Key West. The 40 acre Key West Marine Park has a hard-bottom coral community extending 600 feet offshore on the south side of Key West from Higgs Beach to South Beach. It is a designated swimming-only zone marked by buoys. Unfortunately, the snorkeling here is not known for good visibility and the water clarity is usually between 5 to 15 feet.
At this shore-accessed dive site, the best snorkeling is located just offshore from the intersection of Reynolds Avenue and Atlantic Boulevard at Higgs Beach at the old sunken pier. This section of the park is known for its bio-diversity with over 50 species of fish alone in grassy sand flats. While not known for big coral formations, these small soft and hard coral formation provides habitat for barrel sponges, starfish, sea urchins and queen conchs amongst the sea grass beds. You may also encounter yellow butterflyfish, nurse sharks, grunts, hogfish, damselfish, wrasse, porcupine fish, trunkfish, parrotfish and the occasional spotted eagle ray. Keep on the look out for Portugese Man Of War and for cone jellyfish (non-stinging).
There are several other sections of beach in the Key West Marine Park. However, they are not really known for good snorkeling and are more just for swimming.
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.
Note: The Key West based Reef Relief organziation is responsible for the Key West Marine Park and for marking the major Florida Keys snorkeling sites with over 100 buoys. Consider donating to this worthwhile non-profit organization.
Overall, Key West Marine Park is the 1st most popular snorkel dive spot of all 40 snorkeling dives in the Florida Keys.