Audio frequencies are the only reliable for under water communications. Yes the US Navy used to communicate with rf @ 17kc, but they had a long trailing antenna. The transmitters on land were pretty high powered, in excess of 50kw. Some sonar used 17kc. The transmitting and receiving transducer was a high power speaker with a rubber diaphram. I say receiving and transmitting as the sonar was used to detect ships and other objects in the water . When the transmitted pulse bounced off an object the returned echo was received on the same transducer. I was once aboard a ship that had a fathometer that measured the depth of the mariana's trench, about 6000 fathoms, the peak power of the
fathometer was less than 1kw. One of the problems with underwater communications is that temperature layer's can cause the audio waves to bend.