The family and I love diving and snorkeling at Sombrero Reef. It is in the middle keys, Marathon area. We've used Spirit for snorkeling and Tildens for diving and SNUBA
One of the great things we love about the keys is taking advantage of all the free, very good shore snorkeling areas. The tangs, angels, butterflies, parrot fish etc, can all be seen along the rocky shore areas.
Of course you need to visit the off shore reefs to really appreciate the beautiful fish and corals. Pay attention to the weather forecasts and try to go on a day with great visibility.
Enjoy it