One of a kind is another privateer contract found in Assassin's Creed III Remastered. You can view the walkthrough in the chapter below.
Optional objective: Sink a frigate.
After the mission begins, sail to the green point and eventually you should see the first enemy ships. Your mission will be protecting the channel from the British. You need to survive for the time shown in the top of the screenshot.
To avoid bigger damage, try destroying the enemy ships before they can get close to you. During the fight you will also have to look out for the nearby rocks. The channel is rather narrow and you might crash the Aquila if you won't look where you're sailing.
Eventually a bigger frigate should appear on the horizon. To complete the optional objective, you will have to destroy it before the time runs out. Just like in the previous missions, you should consider upgrading the heated shot which will make destroying it much easier.