We made reservations the day before we took the tour. The first day we arrived at Key West late in the afternoon and managed to find a motel for $69. Since we checked in at around 7pm and the manager was expecting more guests, he switched us to a suite at no extra cost. He also adviced us to take the fastest boat to Try Tortugas. As I recall, the name was "FASTCAT". We made a reservation for two seats at around 8pm. The next morning we left at around 6am and landed on the island at around 10:30am. The return time was around 2pm. So when we got back to Key West, we still had enough time wondering around and took some pics in front of the residence of Ernest Hemingway as well as the southernmost point.
My advice to you: find out about the reservation phone number and make the call one day or two days before your trip, when you are certain about your schedule. We were there in June, the off-peak season of Key West. You may want to make decisions a little bit ahead since right now is the best tourist season of FL. The boat could be booked up very fast.
One last thing, take a disposable waterproof camera with you. If the sun shine is strong enough, you can catch some beautiful pics of the corals and fish! Wish you a wonderful trip to Dry Tortugas! Enjoy!