Sound and radio are the two communication methods used under water. Sound waves carry further and faster underwater than they do in air, although they are distorted. however, rather like dolphins, it is possible to use ultrasound to transmit signals underwater, where they can travel for nup to 40km. The acoustic waves of human speech are transposed in to the ultrasonic range and transmitted from submarines by means of underwater loudspeakers. The recipient obviously have to turn the signals back into human speech. Short radio waves, on the other hand , are blocked by water,which damps anything in the VHF and shortwave ranges.
Long radio waves- with a wavelength over 100m- can penetrate several dozen meters deep in to ariels, which would give away a submarine cannot get to the surface to use ordinary radio for communication, a message can insted be recorded and attached to a buoy that is allowed to surface independently of the submarine so that the signal can be transmitted.
