This will likely be your first achievement in the DLC. As you progress through the first location, you will come to a back yard with the plank over the side of the ship, there's a pirate on it pushing a human over. Just get to the pirate and hit him a few times to push him overboard, too. Achievement unlocked.
If you miss the opportunity, just go back to the MedBay and the pirate will respawn on the plank. (Thanks mCaPP09 for the clarification).
(if you don't know how to access the dlc, you will need a LIFTER HOOK - so progress through the main campaign enough to get it, then head to Port Nixon, down the exo lift in the Iron Maus store and in the open area near water you will see an orange boat. It will take you to a small islet from which you need to use the lifter hook)