Anything below 75 and I am wearing a drysuit and I live in NY, but am originally from Florida. Especially in the winter air. If it's too cold for my 3mm full, I go straight for the drysuit unless it's 90 degrees outside. Last year I was in the Keys and water temps were about 73 and I froze in a 3mm full with a 6mm hood and reef gloves. We didn't stay down for more than 40 minutes. I suggest a 7mil without the hood or gloves for the springs and they are 72 yr round. If you like to maximize your time underwater, then wear more insulation. If travelling to Fl to do a 40 minute dive in 50 feet of water is okay, then wear the 3mil. Ofcourse, many factors can change my opinion like current, depth, wind, rain and the like. I personally like about an hour of bottom time in shallower water, especially if I am paying for a boat trip and travelling to get there.
EVERYBODY will generally be a little different but I think most people get colder than they typically admit or remember. We are all human after all. Don't forget, if the water doesn't kill you, a windy SI might finish the job so bring a nice hooded sweatshirt and get dry between dives, if possible. 3 years ago my wife and I were in Miami and a winter storm hit with temps around 20 degrees and less adding the wind. That was January. Boat diving is cool in the winter, even in S. Fl. Just some thoughts...Have fun!!