You will receive this officer quest at the end of the main quest This Old Cove, when your quartermaster, Adéwalé, informs you about your issues with the lack of a shipwright and tells you where to find a person suitable for the position.
Rescue Lucy from the Prison Ship.
Head to Salt Key Bank and dock the Jackdaw in the local port. The time of day will automatically change to night.
Hint:
Jump into the water and swim to the ship anchored nearby.
You need to reach the hatch leading below deck located at the rear of the ship and the deck is guarded by several soldiers. To do this quietly, climb onto the bow of the ship, but do not step onto the deck - hang over the ledge instead and whistle to lure one of the guards over, then assassinate him.
Do the same with the guard on the other side of the ship.
You can use this same method for the guard patrolling the deck near the hatch below deck.
Climb onto the deck, hide behind the crates, and whistle to attract the brute standing near the hatch.
You do not need to kill the guards at the stern - simply sneak up to the hatch and go below deck.
The rooms below deck are guarded by five soldiers. Move to the right and hide against the wall. Wait for the two talking soldiers to part ways. The one on the left will not notice you, so you can ignore him, while you can sneak up on the one on the right and assassinate him.
Go further and hide behind the crates. Wait for the next two soldiers to finish talking and for one of them to head in your direction - he will likely spot the body of the soldier you just killed and walk over to inspect it. Assassinate him when you get the chance.
Move a bit further, hide behind the shelf, and whistle to attract the second of the two soldiers who were talking a moment ago.
The fifth soldier is standing by the cell talking to Lucy with his back turned to you. Sneak up behind him and assassinate him.
Open the cell where Lucy is trapped. A short cutscene will trigger, after which you will automatically return to the deck of the Jackdaw.
You will be attacked by three enemy ships: two schooners and a brig. You must sink them all using the weapons at your disposal.
Once you do this, a cutscene will play in which Edward encourages Lucy to join his crew but she will not join just yet. This quest will end and the next one will begin automatically - The One With the Gun.
