As soon as you, uh, escape the water, you'll get this.
*** Spoiler - click to reveal ***After the water chases you, and you jump over the holes until you get to the final piece of stone, the water will propel you up to the next ledge. The achievement will pop then.
If you're stuck, here's a visual walkthrough that someone made a while back.
http://www.visualwalkthroughs.com/anotherworld/anotherw....htm