The boat is not only sinking, but Ramses’ men are not about to let Drake get out alive. Take out the enemy on the upper level and when the armored guy comes into view focus on killing him with a grenade launcher if you have it or standard grenades. Next a riot shield enemy will come around and if you run up and melee it you can take cover on the opposite side of the wooden crates to avoid the snipers that pop up behind you. More enemies will pop down from the sniper location, including another armored unit, and eventually the whole boat will capsize.
Head for the exit, marked by the red light, and when you reach the stairs Drake will get fall and bust open a bulkhead door. Jump over the yellow stairs for a scene in which Drake is trapped underwater and tap Triangle to get out before you drown. Back underwater, head for the vent where some light bubbles are coming from to create an exit.
In the next area you’ll need to run through the hall while avoiding the gap and then use the red wire to climb up and shimmy left to use the doors to reach the bar area from earlier. Since the ship is sideways Drake can’t reach the next room without using the bar as a stepping stool to reach the hanging door. In the next room make sure to climb into the bathroom for the Silver Rococo Napkin Ring before climbing up the bookcase and exit through the door.
Climb up to the deck for a nice view as you run along the starboard side of the ship towards the ballroom and then make your way to the elevator and enter. In true Drake style nothing goes according to plan and you find yourself hanging from the chandelier that now lays on the glass roof of the ship. Make your way down the side of the chandelier and jump to the red wire in the middle and then swing to the section that fell. Finally on the floor, head for the exit for a scene and then run for your life to complete the chapter.