Chapter 6 - Slums of Kowloon Level - Debris Field
As you progress through the level, you will come across a firefight (after the cutscene in which one of the buildings starts to blow up) with a mass load of enemies. There will be another cut scene in which a couple of enemies bust open a wall. Kill these guys and you will come across a higher walkway and the wet, sewer bottom. Walk and jump across the path as quickly as possible because there are some parts of the floor that will collapse and cause you to fall if you are not quick enough.
The video below shows the path and when to jump and walk. If you fail and fall down into the water, restart checkpoint and give it another try.
Credit to BarthaxDravtore for making the video.