just across the "dotted line".
I can't remember the exact amount of distance from Ft Jefferson (the "center" of the Tortugas"), but I'd guess maybe 5 to 10 miles. easily doable with short boat ride from an overnight anchorage in the lee of the fort. You could see the lighthouse on Loggerhead key most of the time, but it was a long ass way off and you really needed binoculars to see it some of the time.
They mark the perimeter with bouys I think (not 100% sure), but of course they are spread out spread out.
(I"ve never had to pay attention to where the buoys/boundary is.... you depend on the captain for that.
most of the profiles were I think 50-60ft to 100ft. Might have had one of two that had HUGE coral heads that rose up to maybe 45ft maybe.
of course all of this depends on what sites they take you to. I doubt they hit the same exact sites over and over again each trip based on lots of variables.
as a note... I think there are two official boundary's. I think one (the inner one) is the National Park (Ft Jefferson) and the other is the Marine Sanctuary which extends further out. Ft Jefferson is run by the NPS and the Marine Sanctuary is run by NOAA maybe? (not 100% sure). Someone please correct me if I'm wrong on this....
EDIT:
Found some maps for you. (posting links because they are too freaking huge to do as pics here) they should open in a new window if you click on them.
This shows the boundary's of the Ft Jefferson National Park
http://www.closertonature.com/maps/dry-tortugas-map.jpg
This shows the boundary of the Dry Tortugas Ecological Reserve. It also shows some of the boundary of the FL Keys National Marine Sanctuary too.
http://floridakeys.noaa.gov/graphics/maps/tortugas.jpg
While I'm at it... here is a list of the mooring buoy's. (dive sites). Note that they dive other places besides these buoy's. Most of our dives were on sites with no buoy and were a drift dive.
Mooring Buoy Locations in the Tortugas Ecological Reserve
Have fun. Hope this is helpful to all.
Mike