Once you finish with the water part you'll walk into an area that has crushers that come down when you cross the blue lasers. At the beginning of this part is an area that you can climb up once the crate falls down. Go on the crate jump up to the ledge on the right then jump to the left into a hidden area.
(When using chapter select it's the 24th picture)
You can see it in the video @ 5:00
For a quick walkthrough guide of the achievements check out my video: