The depth a RF signal reliably penetrates to in water is proportional to its wavelength. Hence the ELF sites used to communicate with submarines - we're talki

The depth a RF signal reliably penetrates to in water is proportional to its wavelength. Hence the ELF sites used to communicate with submarines - we're talking EM radiation at a frequency of a few tens of hertz. These transmitter sites are huge - not to mention the power requirements, which is easily in the MW range - so they could only be used to communicate from shore to submersible. Return comms would have to be by acoustic means, due to size and power constraints in the submersible. Also, the bandwidth of ELF systems is very low - think several seconds, if not minutes per bit - the bandwidth of an acoustic system is orders of magnitude higher. Edit: I had bungled the first paragraph. My apologies.