Dry Tortugas
Dry Tortugas Info
Ferry from Key West Takes 2.5 hours, ~70 miles
Camping overnight first come first serve
http://www.parkcation.com/nationalparks/drytortugas/ferry/
Private boaters can and do arrive on the island whenever they like, and they can take up campsites as well. 10 campsites, if full, there is a grass overflow area.
Weather/Temp
The coldest time of year is January, when high temperatures average 73 F (23 C) and lows average 64 F (18 C.)