Just got back from the Keys. Download OPENCPN.org and the NOAA nautical charts. It's all free. The locals are great on the radio about suggesting anchorages. (and there are a ton of places) Mostly just get out the binoculars and start looking for a bunch of mast.
At Key West I understand the military is limiting anchoring in some of the more popular areas north of Key West Bight so you might check that out when you get there.
We didn't get to spend any time in the Dry Tortugas but we were told they are awesome also. Careful studying of the charts showed a bazillion places I would have liked to check out but there were some time constraints on our trip. "Most" of the people in Key West were awesome. There's a boat yard at the entrace to Safe Harbor you definately want to avoid though.