[ReM]

From the U boat commander's handbook:

38.) The submerged submarine is most difficult to spot from the plane when all its horizontal surfaces are painted very dark. All other bright objects on the upper deck, as, for example, the insulators of the net wire, must have a coating of dark paint. In case of need, paint which has crumbled, or been washed off during the operations, must be replaced; for is purpose, a quantity of dark paint should always be available during operations.
39.) A submarine painted in this way can only be spotted by an airplane, if the submarine is submerged,
a) when the sun is shining, and the sunlight penetrates the water below the surface; without the sun, the water is a dark mass, which hides all objects from view;
b) when the surface of the sea is not so rough - approximately from motion [sea] 2 to 3 upwards - that the continuous refraction makes it impossible to see below the surface, even when the sun is shining;
c) when the airplane is almost vertically above the submarine. Because of the high speed of the airplane, it is very difficult to spot a submarine moving under water.
The conditions described above - sun, rough sea, position of the aircraft in relation to the submerged submarine - are relatively more favorable or unfavorable for the airplanes in sea areas with exceptionally clear, or exceptionally turbid, water, for example, in the Mediterranean, and in the Baltic at the mouths of rivers. In sea areas where the water is clear, so that it is correspondingly easier to look into it from airplanes directly above, the submarine must therefore submerge, in good time, to a greater depth, in order not to be spotted.