"Air assassinate the Captain off a rope swing" is an optional mission objective in Sequence 10 / Memory 01: Black Bart's Gambit
Walkthrough[edit]
This optional objective requires a bit of timing to pull off. Once you have cleared the watch towers, you will see a rope in front of you. Wait for the Captain to emerge onto the deck of the ship. Once he has, swing out using the rope and as Edward lets go, initiate the Aerial Assassination. It has a very narrow window for you to execute before Edward just lands on the deck of the ship.