ELF
ELF frequency range (3–300 Hz) (see also SLF) can penetrate seawater to depths of hundreds of meters, allowing communication with submarines at their operating depths. Building an ELF transmitter is a formidable challenge, as they have to work at incredibly long wavelengths: The US Navy's system (called Seafarer) operated at 76 hertz,[1] the Soviet/Russian system (called ZEVS) at 82 hertz.[2] The latter corresponds to a wavelength of 3,658.5 kilometers. That is more than a quarter of the Earth's diameter. Obviously, the usual half-wavelength dipole antenna cannot be constructed, as it would spread across a large country.
Which does use a bouy system to relay between above water communication Satelites, boats, etc... This bouy is not attached to the submarine, earlier designs did. They call it the Deep Siren project.
Blue Laser for Submarine Laser Communications (SLC)
The objective of the Blue Laser for Submarine Laser Communications (SLC) program is to develop and demonstrate duplex SLC that meets a spectrum of bandwidth requirements, to include low bandwidth missions (e.g., anti-submarine warfare) to high bandwidth missions (e.g., Special Operations Forces support or intelligence, surveillance and reconnaissance missions). Submarine speed and depths during the demonstration are expected to be unrestricted. The demonstration results will be used to highlight the increased capabilities of the submarine force to support Navy, joint and coalition operations, and to validate system performance estimates. Program success will transform the submarine from an intermittent node (periscope depth) to a persistent cooperative node (networked at speed & depth).
This has been test at 3mb/s upto 24km point to point underwater. This is reduced to @ 100m +10m when sending signal from submarine to above surface points.
I think this above point is something everyone has been missing...most information is talking about communication to/from submarines to above surface points, very few talk about point to point underwater. This changes how things act, as the signal isn't changing from medium to another to get to where it needs to go. Remember Whales and Dolphins use sonic pulse to communicate 100's ok km.
I believe if you look at it that way the ranges of ELF, SLC and other tech in development could get the ranges wanted, sure there will be limitations due to enviromental affects the same as Wireless communication over land.
Just something to think about!