The north-central region is filled with natural springs, sinkholes and caves. The Tampa bay area doesn’t have any coral reefs, but there are several artificial reefs. Venice Beach, is known for its shark teeth and Crystal River for being the only place in the US where you can dive (actually snorkel) with Manatees.
With such a long coastline, there are of course many other dive spots, like the Fort Myers region. However, the diving is less easy (choppy seas, current, longer distances to the reefs), so it is not covered yet by TheDivingGuide.com.