I used a 2.5mm in July in Key Largo and it was way too warm. I did 4+ dives a day and was very comfortable with just a skin. But then again, there were some people with 5mm and hoods, so I guess everyone is different. Florida Keys Dive Center in Islamorada does dives to the USS Spiegel Grove and I enjoyed my dives with them.
Have fun, I love diving down there.