Everglades National Park
Although you can snorkel here, this park is important to divers not for the diving it provides but rather because it is vital to the life of the coral reefs. The Everglades National Park protects the everglades, which are a major part of the life cylce which includes coral and reef fish. It's the nursery of the Florida Keys, providing a home to baby fish and invertebrates, and it provides the necessary flow of water important to sea grass in Florida Bay. Sea grass of Florida Bay is important to the live of the reefs.
The park does make a good half-day trip for divers taking a break from scuba. If you drive, enter through Homestead, and there are trails to explore, birds everywhere wading in the water (it's a very popular destination for bird-watchers from all over the world, actually), and you might see an alligator. Flats fishing trips can be chartered from here, and there are a couple of visitor centers inside the Park.
You can enter the Park from the water, too, on the bay side of the Florida keys. There are lots of good snorkeling spots, and lots of fish.