If this thread isnt moved there really soon, i would advise you ask in the Florida Conch Divers section. To answer your questions:
Temps: 72F year round
Exposure Protection: 3 or 5mm, depending on your ability to deal with 72F water
Viz: depends, can be 50ft, can be 10ft (depends on the snorkelers around the manatees)
Where to go: try Bird's Underwater, American Pro Diving, Manatee Central, our own CaptSuzieHolstead (board member) or just look/search for threads on the subject in the FCD section - plenty of people have gone to snorkel with the manatees and maybe do the cavern dive afterwards.
Eating: on our next trip, next weekend, i think we will meet and probably eat at a place next to the Best Western, something like ...... on the water is the place, otherwise there are only a few little places.