Submarine Masts Sea Power
Most common submarines have masts to raise above ocean surface, these are periscope, radar mast and ESM mast.
Periscope is for visual observation and identification of surface targets. You cannot use periscope to spot submarines or other underwater objects. Please note that in Sea Power you as a player do not have a direct control for 3D view of the periscope, you can just raise and lower it.
Radar mast is a radar emitter raised above the ocean surface which can detect ships. Using radar mast broadcasts your location to everyone with a ESM receiver.
ESM mast is a passive detection method to surveil all radio transmissions on the surface of the ocean.
Submarines cannot raise masts including periscope on stormy seas, when sea state is very rough. See below example image.
Here soviet submarine attempted to raise ESM mast but could not; Unable to raise mast, sea too rough, out.