Please keep in mind that Florida is a really looong state.
It's about 400 miles from the Georgia border on I-95 to the top of the Keys. I-75 is almost 500 miles long, partly because it also goes E/W over to Miami. And you're still short of the Keys.
The Keys highway is 100 miles long itself and traffic can move really slow, especially during Spring/Easter break. If you go there, try not to hit it on a Saturday or travel back north on a Sunday.
My suggestion would be to find a CG/RV Park somewhere up around Homestead/Florida City and use that as a base for Keys exploration. Florida City is just a hop, skip, and a jump from the upper Keys. Then go to my favorite Florida state park - John Pennekamp Coral Reef State Park and take one of their snorkel trips. The park is at the northern end of the Keys so would be quickly accesible. You can rent all needed snorkel gear, including wet suits, at the park. Just reserve EARLY and be on time (1 hour prior to departure) or you don't get to go ! Also, best to head into the park early in the day because they close it when they reach capacity. I have snorkeled there during the latter part of Feb and water temps were fine, although I was in a shorty wet suit. And . . I have seen Iguanas walking around that park. The little Geico lizards (brown or green) are all over Florida - more so during hotter weather.
If in the Florida City area, you are actually closer to John Pennekamp then if you were staying in the middle Keys for example, especially if you consider driving times.
You would also be right next door to Everglades National Park, which has more gators then you can count.
Don't forget rule #1 - You must pop in the Jimmy Buffett CD when you start down into the Keys.
*** Turtles - Visit the Turtle Hospital in Marathon. Google it.
