The Florida Keys are made of marl, a close cousin of coral; they did not exist then. they need ocean water to form, dry then nothing happens; Cuba and the various islands are a different story.
Nature’s climate pendulum is constantly swinging between warm and cold spells (glacial and inter-glacial periods.) For some unexplained reason, about 100,000 to 125,000 years ago the earth’s temperature began cooling form one of its very warm periods and the two polar ice caps began to grow. This warm period is an interglacial period and this particular is know as the Sangamon period. The increased ice in the polar caps very slowly removed fresh water from the oceans. As a result, the ocean level dropped, the living reefs began to die, and, with time, became completely exposed - Voilia, the Keys appeared. As the sea waters fell, the shoreline moved outward and more land mass was exposed.
The ocean level is thought to have fallen as much as 300 to 450 feet at its lowest levels. The high seawater level occurred about 125,000 years ago, the low was about 20,000 years ago and the outer reefs went under water about 6,000 years ago. This shown in the drawing at the right: The blue is Florida 125,000 before present (BP), the pink Florida 26,000 BP and the gray is today. What today we call the Keys were the very high points of a very wide penisular.
http://www.keyshistory.org/keysgeology.html