Assassin's Creed: Freedom Cry

This may take a while but will most likely unlock through natural gameplay.

In order to plunder a ship, you must first weaken it by using your cannons, mortars etc. Once its health has been depleted down to the red zone, approach it and board it by holding , if you continue to to attack it it will sink and you won't be able to board it. You will then need to defeat a certain number of enemies on board the ship for it to become 'plundered'. The smaller ships will only require you to kill 5 enemies but the much larger ships may require you to kill 15, along with numerous enemy scouts.

Just keep attacking any ship you come across in the ocean until you have plundered 30. It's worth upgrading your ship as much as possible, as this will raise both your attack and defense. You can upgrade your ship at any Habourmaster location such as the one at the Maroon hideout (308, 188).