Question

what do you think briefly about the relation between the initial electric field at the base station and the received field at the submarine to achieve a communication.

Answer 1

Reflection and refraction changes the electric and magnetic fields at a media interface. Seawater shields submarines from most higher frequency radio waves because of its electrical conductivity and also its significantly higher dielectric constant with respect to air, there's a very large reflection at the interface. Very long wavelength radio waves are needed to reach them, requiring extremely low frequency signals (ELF). Shorter wavelengths do not penetrate to any significant depth.