Research Base - Large Room with Water Beneath the Floor

Dive down and swim through the hole in the wall into a pool beneath the floor of a large room.

There are 8 Trinity soldiers above. You must kill them all in order to escape. Start by swimming forward to the large opening just ahead. Hold LT/Right Mouse to stay underwater until there's only one soldier standing above the opening.

When you see the Melee icon above his head...

...press Y/F to drag him into the water and drown him.

There are several holes in the ceiling, and the room above is full of smoke. Use this to your advantage. Pick a hole somewhere on the other side of the room, swim toward it, and surface cautiously.

If you see a red triangle icon above...

...this means an enemy is aware of Lara's presence and will shoot her if he gets line of sight. So don't climb out there. Instead, quickly change direction and choose another opening. Remember, you have the rebreather, so there's no hurry. Take your time and stay cool.

When you find a safe opening, climb out of the water.

As the soldiers mill around looking for Lara, you can track them by their laser sights...

...or use Survival Instinct. Take out one or two with headshots.

Then dive back into the pool...

...and swim to a different opening.

Climb out of the water and move behind cover if there is any nearby.

Repeat the same tactic several times, killing one or two men each time you surface. Or, if you see 2 or 3 soldiers clustered together...

...use a poison arrow to take out the group.

If you want to search the enemies' bodies for salvage and such, either do it quickly as you go or leave one man alive at the end. As soon as you've killed the last Trinity soldier, a cutscene kicks in and the level is over.