In the north part of the map you can find Major Volker Grün. The Major is also a optional objectives and so nearly unmissable. Beside the Major is a boat located. Before you can do this achievement clear this boat from all enemies then kill the Major Volker Grün, don't forget to loot the dead body (collectible!) from Major Volker Grün. Pick up the dead body and go to a end of this boat and drop the dead body into the water.