Key Largo
The northern most key of the Florida Keys, Key Largo is just a short drive from Miami.  You can scuba dive wrecks like the 500+ ft. Spiegel Grove or snorkel ther famous Jesus statue.  Many dive charters and stores are readily available to make your dive a pleasurable experience.

Key Largo is also home to the John Pennekamp State Park.  A large, preserved, underwater reef garden.  The state park offers snorkel tours right in the park.