Navassa is an island, and as such you can circle around it and get in shots on all 9 of its defenses. Unfortunately, the schooners that protect this fort can easily pose a threat, matching speed ahead of the Jackdaw placing Fire Barrels. The schooners should be your first target, taking them out at a distance from the fort to make a safe approach and allowing you to board them for repairs if you take too much damage.
Once you are on the ground, enter the fort via the lift straight ahead and then climb the left wall to avoid the eyes of the Captain off to the right. Head straight and get on the building nearby to air assassinate the first Captain before he runs off to the south. You can now climb up the building again and air assassinate the Captain in the courtyard on the east side of the building. Head back up the steps and then run through the scaffolding and air assassinate the final Captain to enter the nearby war room and kill the fort's Commander.
After the fort is cleared you can get the Treasure x2, Animus Fragment x2, and start the Naval Contract, Blind Justice.
