Render error due to faulty element

Hi guys,

I Finally got an excuse to visualize something - this time a beach scene :)
But oh no, the water is messing up!

Update! Problem is solved: Turned out to be a large mesh-from-linestring that was fuxxord. The mesh was not even visible in the scene!

The rest of this post is kept for future reference :)

Here is a Workaround to fix my error:

1. Made new model in DGN
2. Referenced faulty project (Still rendered badly)
3. When copying parts from old model to new model and then rendering it rendered ok

Conclusion thus far: A faulty element somewhere is messing up the rendering.
Solution: Keep copying elements untill it fails.

ORIGINAL PROBLEM: Why the dark area?
What happens is this; Just below the water surface geometry there is a dark area rendered on the geometry below.
It didnt allways do this!

Look:

The water is a simple flat shape using transparency with a "falloff" effect called "absorption distance" under the transparency tab. Also reflections/bumpmas and such. The Sand and stone (yeah its stone LOL.. Im no Kurt S.. yet) are meshes with pretty basic materials.

Rendering with a sun + gradient environment.

Thanks in advance for ANY ideas,
/Torben

Tried amongst other things:

- Shadow toggle on water shader, other rendering setups
- Various rendering settings