Originally Posted by Hitman

The amoutn of visibility for each fog state is encoded in the scene.dat, and you can tweak it with Silent Editor easily. But it is not an absolute number, instead it is a % of the maximum visibility. Hence it will depend on which environment mod you have installed.

Of course you can make pea soup appear, in fact it is already encoded for the worst fog state. Only it will appear in seldom conditions, and unfortunately not tied to calm seas.