If you like snorkeling and if you're staying in Marathon key, go to sombrero reef. Looooots of fish. Not to be compared to the Great Coral Reef in Australia but still very nice. It's the 3rd largest coral reef. The 2D is in Belize. We rented a boat for half a day (8 till 12). 30 min to go there, 2 hours snorkeling and back.
Absolutely worth it, we all loved it.