Land Shark Trophy in Uncharted 3: Drake's Deception

To defeat an enemy whilst swimming, you must be in a body of water. I know it sounds obvious but for the sake of clarity, hanging from a raft or barge will not count as a kill despite pretty much all of your body being underwater.

Chances to pick up this trophy are very few and far between and the only real opportunity is in Chapter 12 - Abducted right after you climb through the ships internal chambers to burst out of a ship door which overlooks a giant sort of ship 'graveyard'. Enter the water and stealthily work your way around to the first enemy to take aim, the bobbing will affect your aim so get as close as you can and down him. You will blow your cover of course but swim around the area taking out as many enemies as you can, there will be a turret at the far side so when you get that far, there wont be many chances left. In order to maximise your ammo, aim for the numerous propane tanks on the gangways and rafts as well, these are easier to go for and the blast damage can often kill an extra enemy too.

Two playthroughs should be enough to bag this if you kill enough enemies from the water but if not then you can replay this chapter and take out as many enemies as you can before saving/reloading and starting again.

Upon killing the 20th enemy from the water, the trophy will unlock.