5' of draft
is hard to get inside without high tide and local knowledge. You can anchor
on the lee side of the islands.
The east side has as a lot of current
and the north side is shallow pretty far out. The south side or the west side are better for anchoring
. The west side makes a great jump off for the dry tortugas
because it's an easy place to leave from in the dark. I strongly recommend you make the tortugas in daylight, which may mean leaving very early in the morning depending on your boat speed.