If you use minitweaker.
Use the Scene.dat tweak file with minitweaker... point the programme to your scene.dat file.. then proceed to this line...
[3]
DropDownName=Underwater
absolute,single,3DE,FresnelCoef
absolute,single,3EA,Fog start
<------ Maybe even reduce this.
absolute,single,3ee,Fog end
<------ Reduce this number.
absolute,color,3FC,UpLight (color)
absolute,color,418,DownLight (color)
Fog end is where the fog becomes so dense you won't see anything. Sorry if any of this is incorrect, it's been a long time since I used minitweaker.
More resources here.
http://www.subsim.com/radioroom/showthread.php?t=110889
Also, I do not know if this will have an affect while viewing through the water surface. It may only have an affect while in underwater view.