tartamimmi,
I finally got some time to look at the cancellation more closely. Here is an image where I isolated the red channel of the image:

I still cannot figure out exactly what it is. It is possible that the cancel is not words. However, I do see a possibility that the lowercase letter "n", "m", or "r" is in the middle of it.
I have read the history of this postal location which you posted in this thread. I tried to find any correlation but cannot find any.
Maybe, the cancel is not from the Tortugas office and could be from somewhere else on the Gulf of Mexico or eastern Atlantic coast close to Fort Jefferson.
I wish you luck in finding the meaning of the cancellation. Please keep in mind that the cancellation may just be a random shape and does not represent letters of the alphabet.
I still think that it is composed of letters, but I cannot determine what it could possibly say.
Good luck!