As you'll progress through chapter 6, you'll have to use a crane to chuck enemies into the water. When you use the crane the second time, there will be several stacks of pipes in the area, three on the right and one on the left. Pick these up and toss them into the water and the achievement will be yours.
One big thing to note though, be sure to get rid of the pipes before the enemies, as once the last enemy is chucked in the water, the crane sequence will end.