West of Fort Myers, the snorkeling at Cayo Costa State Park is popular, but not known to be good. The water clarity is quite poor most of the year with one to five foot visibility at best. The highest water clarity can be found in April and May at hightide during periods without rainfall and on low-wind / low-surf days. There is no coral or rocks to provide habitat for marine life, but there are still some around to see, such as sargeant majors. Most snorkelers hang out in the shallows where there are plenty of sand dollars and seashells to discover. If you are lucky, you may even spot dolphins and turtles.
Note: the Cayo Costa State Park is only accessed via private boat or ferry boat. There is a ferry boat service offered twice a day, but reservations are required. The ferry boat only goes if there are enough passengers so confirm how many people have reserved a spot the day prior. Also, the ferry is known to depart early, so make sure to arrive at least 30 minutes prior at the dock. Once you arrive at Cayo Costa, there is a State Park entrance fee payable on the dock, so bring extra cash. The dock is on the east side of the island and there is a free hourly shuttle to the west side. There are limited services on the island. Also, the island is known for plenty of mosquitos, and there are alligators in and around the large lagoon on the southwest side of the island.
To reach the ferry boat dock to Cayo Costa from US75, use Exit #143. Head west on SR78 onto Pine Island. Turn right heading north on Stringfellow Road and follow the signs. Snorkeling on the Florida Gulf Coast, such as at Cayo Costa State Park, is not known for good visibility. Matter of fact, the snorkeling is known for very low water clarity in the range from 1 foot to 7 feet most of the year. If you are on Captiva Island, there are some snorkel boat tours that do come up to Cayo Costa. The snorkeling off Captiva Island is similar to that on Cayo Costa.
Overall, Cayo Costa State Park is the 2nd most popular snorkel dive spot of all 18 snorkeling dives in Florida Gulf Coast.