Counter kill 5 enemies
Head on toward the Sailors that are in front of you. Use them to start toward the Full Synch requirement. Otherwise, Counter kill one and then start into the next with a chain kill. You need to be careful that you get them individually with Counter-kills. You can usually manage the 5 in the first area if you are very careful about it.
Head on over the crates to the right and start into the next part of the ship. Deal with the Sailors and Soldiers you find the area. Be careful going over to the right as you will face a Firing Line. You will also see the Captain start running away.
Dodge firing line then take off after the Captain. Chase him down. Done quickly enough he will not defend himself and you can get a quick assassination on him.