Marco Solo Trophy in Uncharted 3: Drake's Deception

This trophy is unlocked in Chapter 14 - Cruisin' for a Bruisin'

During Chapter 14 you will become enclosed in a battle with a handful of regular enemies and a heavy armour PAK 80 wielding guard, a battle made trickier with the constant banking and listing of the ship which throws the boxes around the deck while you try and take aim. Once you have killed everyone, there is a red door which you can open so go through it to be presented with another portion of the deck with a pool in front of you. There is very little water in it thanks to the violent storm rocking the ship side to side and emptying most of it but if you jump in regardless, you will hear Nate call out "Marco? ...Marco?" and the trophy will unlock.

Here's a video showing where it's done, courtesy of PowerPyx: