This mission is essentially an introduction to attacking and plundering on the high seas. What we need to do is to pull out our spyglass using the button prompt indicated on screen and zoom in and out to identify ships in the area. When focused on a vessel, the spyglass will give you information about the class of ship, what threat level they pose to the Jackdaw, the sorts of weapons they have and any cargo they might be carrying.
Go for a bit of a sail around the area and use your looking glass on all the ships you come across until you find a Schooner. Once you have located one, pilot the Jackdaw over to it and use your weapons to bring it down to low health in order to incapacitate it. Once it is ready to be boarded, a green beam of light will appear from the top of it. At this point, move in close and hold down the button prompt to initiate a boarding sequence.
Once Edward lets go of the wheel, quickly head over to the railing nearby with the swivel gun on it. Hop on the swivel gun and use it to target enemy sailors on the deck of the damaged ship. Kill three of them with the swivel gun to complete one of the mission’s optional objectives. Next, clear out the remaining enemies on the enemy ship to complete the boarding sequence. A scene will play.
After the scene, the game will update your objective marker and ask you to return to Salt Key. Note that to complete the other optional objective; you will need to plunder at least 20 Rum. If the schooner that you looted initially did not have any rum on it, you’ll need to keep sailing around the area and using the spyglass to find a ship with some rum on board. Plunder ships until you have completed the objective.
Following your return to Salt key, dock and enter the town. Make your way over to the Harbourmaster and purchase the intermediate hull upgrade for 1000 Reales. This will complete the mission.