Biscayne National Park
Although not in the Florida Keys, Biscayne National Park is just north of Key Largo and is therefore important to the health of the reefs in the Keys. There are excellent coral reefs in this Park, and you can get some good diving under your belt from a private boat or by taking a scuba trip that leaves from inside the park itself. There are bank reefs, patch reefs, old and new shipwrecks, and islands here. The bay called Card Sound is located within the Park boundaries, also.
The 45 tiny key islands and thick mangroves lining the shore of Biscayne National Park provide a feeding ground for millions of small baby fish, which grow up and come out to the reef to live. Biscayne is worth a visit for divers, and the best time is mid-December to mid-April. There are more than two-hundred species of fish here in the protected reefs of the park, and you can take glass bottom boat tours if you want to take a break from diving or snorkeling. The park is accessible on land from Homestead and there's no fee to get in.