Well thank you! So it was the displacement that was the culprit. But the mystery is not solved, as other terrains with the same material and the displacement not turned of works, but never mind!

After I disabled the displacement and added some minor enhancements it turned out like that (rendertime15 min!),

http://www.renderosity.com/mod/gallery/index.php?image_id=2200506

Observe the water. The water texture has nice foams on it, but if I turn on transparency the foam disappear. The only way to have both (ie to have the cake and eat it) is to make two renders of water, one transparent and one with foam. Then I can in Photoshop exact decide where I want foam and where I want transparency. (not done yet)

 I am using a reference, and the Vue scene is positioned exactly as the ref.