Climb back onto the ledge between the two pools. This time, don't jump across to the stairs on the west wall, like you did when you went after the secret. Instead, move to the right side of the ledge and turn around.

Drop back and grab the edge. Then let go and quickly press Action again to grab the diagonal crevice below.

Traverse along the crevice, downward and to the right.

As soon as you round the first corner, let go and grab the crevice below. Continue traversing to the right, around one corner...

...then another...

...until you can pull up into a crawlspace on the north wall.

Crawl through and drop into the shallow water on the other side.

Turn around, wade forward, and pick up some Uzi ammo. Then pull up into the opening on the right.

You might want to save before crawling out the other side and dropping onto the angled block below. When you drop, hold Jump + Action.

Lara will then backflip onto another angled block...

...then jump again and grab the crack in the wall ahead.

Release Jump but continue holding Action as you traverse to the left until you can pull up into an alcove with a small medipack.

[Citadel Walkthrough]