bruno: IMHO there has to be something else to the grainyness other than too much amb. light or different lighting conditions; I believe that it's a render engine problem; one probably have to increase the AA settings to the limits to obtain pics like above.

I took a fast look at my Bryce renderings and the difference is almost shocking: no grains in my Bryce waters at all (with many different light conditions, simplest AA settings); I never had to think about "grainyness" in water when working with B; I hope Vue can iron out this particular problem.