This contract has us taking on the fort at Dry Tortuga. Now if you have already taken over the fort previously, a restricted area will appear inside the fort’s walls. If you have not yet taken over the fort, now is about as good a time as any to do so. Check out the Fortress section of the guide for a way to take the fort down!
Once you arrive, the target will be hiding somewhere in the green zone. Enter the area and use Eagle Vision to identify him. For me, he was hanging out in the courtyard at the very top of the fort. Once he has been identified, assassinate him to fulfil the requirements for the contract.