A submarine can see an enemy helicopter at a height of 800m above the sea. What is the actual height of the helicopter above the sea. Given water Myu air=3/4

I don't quite understand the question? No light is refracted - it only passes through air?

Draw yourself a picture of a light beam going from the water to the air. The angle wrt the vertical will be larger in the air than in the water because of refraction. When you look from underneath water surface things seem to be higher up because of that than they really are.