Fog values for game engine (visually speaking, not for AI) are in scene.dat file, at least in part.
Changing those values could increase or decrease max visual range. For example:
Smoke in horizon. This is from a ship at about 8.4 km with default game values:
http://www.mp-labs.com/sh2mods/fog01.jpg
Moving the fog start a little more far away we could see this, the same smoke but visible at 10 km:
http://www.mp-labs.com/sh2mods/nofog01.jpg
A comparison of two Gibraltar images from the same place in a clear day:
http://www.mp-labs.com/sh2mods/nofog02.jpg
Gibraltar in a day with medium fog:
http://www.mp-labs.com/sh2mods/fog02.jpg
But it seems that game engine only draws as far as 10 km (clip distance). We can't see any object further, even removing all fog. I haven't found clip distances in any file yet, but changing them could be a good step in increasing the visual distance.
There is a small problem with the dynamic ocean. It only covers about 1000 meters from player's eye. From there to horizon, there is a flat pseudo-ocean that fades with the fog. I can make the dynamic ocean grid bigger easily, but it is practically at the same height that the pseudo-ocean, less than 1 meter of separation. This makes Z-fighting polygons, as we see here:
http://www.mp-labs.com/sh2mods/renderengine02.jpg
It could be fixed by lowering that plane a little.
I supose that the main reason of making a too short viewing distance was to maintain a minimum fps. I know this because making the dynamic ocean grid four times bigger eats about 20% of the fps. Surely the dev team had the same problem and that is why they limited the range.
This is how world is rendered in the game:
http://www.mp-labs.com/sh2mods/renderengine01.jpg
A draw to explain it:
http://www.mp-labs.com/sh2mods/renderengine03.jpg
I hope this would be useful. I don't know about the AI part, only researched how increase the player sight.
I will put the hex addresses that I found, if interested.