WAS' endeavour to make a sub-ice render made me do a little experimention as well. The ice is an inverted, no shadow sphere, colored by illumination, translucency and color, followed by a no-highlight RT reflective shader. The murk was a tricky part, as the illumintaion of the ice also lightened up the murk, which I wanted dark. So it couldn't be too light. Maybe a glass shader with SSS would have worked better, but I didn't try that.
The U99 I once found on the web, but I have no idea who to credit. Sharks neither. Actually, sharks probably don't exist in the Arctic, but well....
Big render took 1,5hour, standard renderer.