If you're refering to underwater clarity then this may help you.
Open the scene.dat with Sh3-editor and look for the following entry:
The highlighted "ZMax=15" is the area that you'd need to tinker with (both ZMin and Zmax). I forget what each of the values correlate to but basically a higher number equates to better visibility. I guess you could trial and error your way to what you want (my values above are for poor visibility and very murky underwater clarity).
I hope that helps!