If you get thirsty or want a snack, you can board the tourist ferry
at the fort and buy food
and drinks. Only thing is you must consume everything on board.
If you stop at Boca Grande, you just need to watch out for the current
. It can be pretty fast. I've seen people try to swim back to their boat from the beach an not make it. Luckily someone was on board their boat and lifted anchor
and went after them before they got too far out to sea. (I was lowering my dingy to go get them when I saw they had it covered).
The times I have been to the fort there have been a good dozen boats there. So you will not be alone. You can easily take 2-3 days to visit the fort and snorkel around it. Let me know if you see the crocodile in the moat.
Be sure to take a guided tour (or follow one from the ferry). What makes Ft. Jefferson interesting are the stories. The stories of the architecture and its failures, disease, the terrible conditions and of course Dr. Mudd.