Originally Posted by Canonicus

"You can get rid of that effect easily.
Open Underwater.pfx in your SilentHunter4 \Data\Filters folder with notepad.
Set the intensity of UnderwaterDispMap to 0.000 .
That's all. You can play now without underwater distortion"... Thanks to WilhelmTell for the answer.

You need the 'underwater disp map' for the water cascading effects on the periscope lens when it comes out of the water...so don't delete it.