Fort Jefferson, Dry Tortugas Natl Park

You can take a ferry or seaplane out to the tiny island that is Dry Tortugas National Park. Here you will find a 19th century Civil War fort that was also used as a prison. The water is crystal clear and shallow, and you can snorkel, admiring the schools of beautiful fish. The fort provides some shade in the heat of the day. There are hermit crabs galore on the island, and a frigate bird colony- you can see their six foot wingspan as they fly overhead and hear their calls (their nesting area is closed off). If you come by ferry or sea plane, you will have a few hours to snorkel and explore the fort. This trip takes some advance planning- you must book about six months ahead.

You can also camp overnight. At night you will see a million stars and bioluminescence in the water, plus hear the sounds of hermit crabs scurrying and palm rats screeching like cats. If you camp, you will almost have the island to yourself. The ferry leaves at 5:30am for those that wish to camp so you must stay in an expensive hotel in Key West the night before (add this to your budget).
About two hundred people come by ferry each late morning, to explore the island. If you come by seaplane you can choose the 2:30pm trip, when all the other daytrippers have headed home. From the sea plane you can see shipwrecks, turtles, and sharks. 
You can take a tour of the massive fort, the largest brick structure in the US, which has an interesting history- read the story of the imprisonment of Dr. Samuel Mudd. Enjoy the vibrant colors of the sun-drenched island. And you can snorkel here, the third largest barrier reef in the world. The outer moat is the best place to find sea life.
The ferry ride is 2.5 hours, and you are provided breakfast and lunch, plus you can buy drinks or bring some extra water. The seaplane trip takes 40 minutes and you have 2.5 hours on the island. 
It is rare to encounter mosquitoes on the island because there is no fresh water. Wow makes me want to move there haha!
There is no running water on the island, just compost toilets. The ferry boat has toilets, outdoor showers, AC, and bag lunches you can buy- you can use these amenities when the ferry is on the island, 10-2 daily, even if you're camping. 
-Thank you Kele Santi for your beautiful photos and travel advice.

Directions

Ferry ride on the Yankee Freedom is $190 for adults, $180 for students/seniors/military, and $135 for kids aged 4-16. Prices are $210 for adults and $155 for kids for camping ferry trip. Make sure to also budget in the incredibly high cost of hotels in Key West. The ferry leaves at 6:30am for day trips and 5:30am for camping. Bring some dramamine in case of choppy water.
Seaplane Adventures costs $361 for adults and $288 for kids aged 12 and under. Kids under 2 ride for free. Morning and afternoon trips.
Book six months ahead for the day ferry and a year ahead for camping.
You can rent tents, sleeping pads, pillows, sheets, blankets, silverware, cups, plates, and coolers. The company, Camping Florida Keys, will bring the gear to the dock. It is run by a former ranger at Dry Tortugas.

Click on map for interactive view