Key Biscayne is the only national park that is largely underwater, so to appreciate its beauty, you really should view it through a snorkel. Fortunately, this private concession located at park headquarters can provide everything you need including snorkeling gear and transportation out to a reef with crystalline water where you can admire coral and other marine life. Canoe and kayak rentals are also available.
If you go:
Snorkeling expeditions are available throughout the year. Winter might get a bit chilly, but it's also less crowded, and the crew is more likely to improvise, taking you to a part of the reef most people don't get to see. Snorkeling trips depart twice daily, provided there are a minimum number of participants. Reservations recommended.
Links:

9700 Southwest 328 Street
Homestead, Florida 33033 USA
BiscayneNationalPark@gmail.com
Phone Numbers:
Information
+1 (305) 230-1100
+1 (305) 230-1100