Well, the best shallow-water snorkeling on the Gulf Coast is on Siesta Key at a place called Point-Of-Rocks.

While 99 percent of the beaches in Florida are pure sand and shell, this spot has an outcropping of flat limestone rock that covers several acres of shallow water. Fish congregate here to feed, which makes for some very interesting snorkeling. The water here is usually exceptionally clear and calm, and crowds are light.

Good luck, and the link below is to a webpage about Florida beaches and about halfway down the page is an article titled: "Best Snorkeling Beach - Crescent Beach, Siesta Key" with all the info you need...