Optional objective: Destroy ships using only one broadside - 3.
As you take control over Connor again, head to the docks and begin another memory.
As you control the ship, you will have to destroy a dozen of nearby enemy ships. For full synchronization, three of them should be destroyed with a single salvo each. It's not all that easy and often depends on luck, but always remember to aim at the smaller units which are easier to destroy.
Optional objective: Destroy frigates by firing on their powder stores - 2.
With most of them destroyed, three frigates will appear.
To complete the optional objective, you will need to destroy two of them with the swivel gun by aiming at their powder stores, marked with a circle.
Eventually you will have to deal with a powerful liner. Start off by ramming it and board it.
Optional objective: Counter kill enemies - 5.
Optional objective: Completed all the constraints in one playthrough.
As you reach the deck, you will have to find and kill the captain. Before you do it however, defeat 5 enemies with simple counterattacks.
In one part of the ship you will come across a group of riflemen. Try eliminating them as fast as possible or hide behind a wall and fight there.
After you kill the captain, quickly leave the ship before the gunpowder explodes. That way you will end the mission.