Southwest of St Petersburg, the snorkeling at Egmont Key State Park is well known, especially around the old Fort Dade ruins offshore. The water clarity is usually pretty low in the range of one to five feet. For the best chance at good visibility, try heading out to snorkel at Egmont Key in late April or May after a period without rain. Shoot for within 2 hours of high tide on a windless morning. The main ferry landing site is on the north end of the island near the lighthouse. There are paths leading south through the island interior.
There are several shore access dive sites. The best two are off the Pilot dock on the east shore that has grass beds providing habitat for starfish and stingrays, and off the old power station on the west shore.
The most popular snorkeling dive site is the offshore Fort Dade ruins. These ruins were previous on land, however, over the years, the shoreline has eroded resulting in the fort foundation being about 150 feet offshore. Since the shoreline on this section of the island is a bird sanctuary, the only means of accessing the Fort Dade snorkeling is by boat. The main ferry on occasion runs an add-on service over to the fort for an hour onsite. Once onsite at the fort, you can snorkel around the entire foundation. The waters are roughly 15 feet deep and you can expect to find plenty of marine life including sargeant majors, snapper and other reef fish. If you are lucky, you may see dophins and sea turtles.
The Egmont Key State Park is located at the mouth of Tampa Bay and is only accessible by boat. The main access to the island is via a twice-daily passenger ferry from Fort De Soto Bay Pier offered by Hubards Marina. This passenger ferry typically departs at 10am and 11am, and returns at 2pm and 3pm (reservations recommended). The round trip cost is $20 for adults and $10 for children. In addition, they provide an add-on service running to the Fort Dade ruins for snorkelers at a cost of $15 for one-hour on-site (April through September). Make sure to reconfirm your reservation the afternoon prior since the ferry service is known to cancel without notice if there are not at least 10 passengers. Ask them exactly how many reservations they have for your trip to get a feel for whether or not it will be cancelled. The earlier trip tends to cancel more than the later trip.
Alternately, there are at least three private excursion charter boats offering Egmont Key State Park snorkeling trips.
Note: Egmont Key State Park does not offer any services on the island, so bring your own food and supplies. While on the island, keep an eye out for the gopher tortoises.
Snorkeling on the Florida Gulf Coast, such as at Egmont Key, 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.