If anyone was following this I wanted to follow up. We ended up not going to the Dry Tortugas.
We went fishing instead. Was able to book via Viator online and redeem with Arrival +.
Wanted to add that I had really good luck with manual redemptions with Arrival +. Fairly confident that you could get this covered with Arrival points.
Good luck if anyone goes there. It is still on our list but we were too busy fishing, jet skiing, snorkeling and other things when in the keys.
Great trip and will go again.