It was noted by Deeeco that on the lowest difficulty there is only one guard in this area. To be safe, play on "normal" or higher when going for this achievement. Thanks, Deeeco!
I did this on the tanker, deck-a, crew's lounge starboard. When you enter on the right hand side of the screen there will be one guy sitting on the steps and another that roams half way down the hall. Stand just outside the opening, wait for the roaming guard to walk away from you and shoot the sitting guy in the head with the m9. When the roaming guard comes back he'll kick the passed out guy. After he does, immediately shoot him (they one who kicked the first guy) in the head. The other guy will get up and see his bro passed out. Simply rinse and repeat. You will need to find more m9 ammo eventually, I suggest saving about 5 rounds, leave the crew lounge and tranq other guys and do the pick up the body and drop it routine until you get more rounds or you can search the rest of the boat. If you happen to shoot the sitting guy when the roaming guy sees it, you can usually immediately run back out and wait about 30 secs or so, they should return to normal. Hope this helps speed things up a bit.