This achievement can be acquired on the third mission of the campaign, Wetwork.

1) As soon as you spawn, swim forwards to the pier and kill the first enemy with a throwing knife.

2) He will drop a silenced pistol. Climb onto the pier pick it up.

3) Swim towards the boats on your right and take out the next guard standing alone on a pier with the pistol. Make sure the swim beneath the surface before opening fire!

4) Swim towards your left, under the moving speedboat and towards the next pier. Take out the enemy standing at the end of the pier. He will be facing the other direction.

5) It is likely this will be observed by other enemies and a firefight will ensue. Kill the last enemy whilst underwater now.


Tips: Don't forget to resurface occasionally to navigate and breathe!
This will be easier to achieve on recruit or regular difficulty.
You can pick up the gun of the second enemy you kill. The original pistol you will pick up has low ammunition.