Hi,
I was just down there (I'm from NY- cold fresh water diver) in Jan 2003 and dove off the Keys. Water temp was 73 degs, air temps were about 74-76 with about a 10-15 mph wind. We wore 3mm jumpsuits and I was warm in the water, however, in between dives I was chilled enough while on the boat to shiver quite a bit. I am fairly lean build, so I get chilled easily. 1 person on the charter had a drysuit and wore it. As soon as I got back to NY, I ordered my BARE ATR Light Tri-Lam drysuit from a Canadian shop. GREAT PRICE!
I have always dove a 6.5mm 2-pc wetsuit up here and it worked well enough - however, kepping warm in between dives can be a VERY real concern. I certainly do not feel that a drysuit is overkill for even warm water temps now. My drysuit will be with me on all future dives, here or in Florida!
Good luck - you'll love the Keys...I did. We'll be moving down in 2 yrs when I am done with college.