In Blender 2.71 using the Cycles engine, I cannot get displacement to work properly on certain objects.
Here are some screenshots of thre three objects and the nodes I'm using for them: water, a cube, and a plane. Although the displacement works fine for the cube and the plane, it refuses to work on the water. The nodes I'm using are based off of this answer.
Water nodes:

Cube nodes:

Plane nodes:

Viewport:

Render:

Here's the .blend. You'll also need the texture files from here and here.


