This achievement should only be attempted on regular difficulty or below as you will be shot by enemies in higher positions while you are underwater. There are also only going to be around 6-8 enemies in the water so there isn't plenty of them to kill. The first kill underwater will be after the flood when Rorke helps you. Just after this you will drop down into more water and there will be several enemies, shoot the enemies that are not in the water and then crouch and knife the two or three that are. Just after this you will cross the flooded road on top of a bus and you can crouch underwater and flank around the left and stab the two or three enemies that are in the water.

This doesn't really seem like too hard of an achievement to get. I'll definitely be going for it in the near future

There are three different locations during this mission where you can kill enemies while underwater. The water is just waist-deep, so you have to crouch and then kill enemies with your knife. You should definitely play on the easiest difficulty "Rookie" for this. Enemies will still be shooting at you when you go underwater and you have to get really close to them to perform knife kills.
Just kill everyone standing in water. Run up to them, crouch and use the knife.