The National Weather Service maintians a weather station at Fowey Rocks, just off the coast of Miami Beach.
Visit http://www.ndbc.noaa.gov/station_page.php?station=FWYF1 for current air temp,water temp, and wind speed.
The official South Florida Marine Forcast is available at http://www.ndbc.noaa.gov/data/Forecasts/FZUS52.KMFL.html
Generally our water temp is quite comfortable allowing us to dive in the summer with at most a lycra skin suit. Winter months you may want either a 3-5 mm wetsuit. You can always call for a recommendation at time of booking your trip.