Yeah, I am still trying to figure out what Vixen means by that. Radio can work underwater, but only at proximate range. Otherwise you are toast as far as comms are concerned. Diver to diver? Maybe if they are proximate. Diver to ship? Not happening. It's wired comms or nothing. Not sure why some folk cannot get their head around the fact that oceans are an awful lot denser than air, but the answer may be in the question. It is all about density.
Likely, ELF will come up. That could provide two way communication, right? Nope for a shedload of reasons. Anyone could look up why ELF remains a one way transaction if they were sufficiently motivated. A shore base has no real problem wheeling out a kilometers long antenna. A sub does. Besides, the comms on such a channel cannot support voice. And ELF cannot sustain voice comms. Not even typist speed. Typists operate in the realm of words-per-minute. ELF operates at the level of letters per minute. It ain't fast and it ain't voice.