After the conversation, approach the mast and climb up the rope. From there you will have to get onto the second mast and climb to its top. From there you should see two ships fighting.
When Connor returns to the deck, approach the wheel and sail in the designated way. To reach the attacked ship, you will need to navigate through numerous rocks. It's not an easy task during sharp wind, so remember to go half sail during strong gusts.
Optional objective: Limit damage from rogue waves - 25%
After you get out of the rock labyrinth, rogue waves will start hitting the ship. Always face them with the board and press the attack button right before they hit to avoid damage.
Optional objective: Limit the number of missed shots - 2
As you reach the merchant ship, try destroying all enemy boats as fast as possible. However for full synchronization, you can miss a shot only once, so be very careful and patient. Be sure to take the waves into consideration, as they can block the path of the cannonball.
Optional objective: Destroy frigates by firing on their weak points - 2
After the battle, keep sailing in the designated way until you get attacked by two frigates. For full synchronization, you need to approach them and hit their weak points with the swivel guns. Blowing up the second ship will end the mission.