Hi Ruta.

It's when it was set to Volumetric Sunlight that I had all the Bucket marks in my image. When I set it to "No shaddows on clouds" the problem went away. Quite odd really.

If you like you can download the files I sent to e-on for testing.
http://www.irishacts.com/water.zip
The zip contains the model, scene and the first image posted at the start of the thread.

Kind Regards
James