A cant miss dive site off Marathon, is the Thunderbolt. Excellent dive, about 80 feet of water. While you are in Marathon, Sombrero Reef is an excellent Spur and Groove reef. Not too far from Marathon is Looe Key, perhaps the best protected reef in The Keys. The best bet here, is to dive very late afternoon, for the best marine life. If you do ever want to dive the Gulf Coast of Florida, the best dive site is the Florida Middle Grounds. Tons of big fish, lots of deep water corals and tropicals. This site is an advanced dive, as it is 80 to 170 feet, and a 80 mile ride from shore. We usually fish and dive these waters several times a year. I have dove all over Florida and the Carribbean, and the middle grounds is my number one dive destination. BTW.... your best bet is to do the Florida dives in Late April or May. Best time for dive conditions.