Btw. the grain (in water especially, but also in mist itself) also bugs me. It's a very shallow (80m) v2 mist layer, with cloud quality at 2, AA=6, robust sampler, but with noise threshold changed from 0.05 to 0.04. Still not good enough, apparently. I have to experiment with that too. The ray detail multiplier was set at 0.5 instead of default 0.25.
I agree, but I hoped to find a fast and less grainy solution.