KMD, the DC to 30kHz RF spectrum is pretty much the property of the USN simply because the USN spent the money to build the antennas required to operate in that region of the spectrum efficiently. There may be some other nations who have built transmitters in that region. I don't know if Canada has or not. Doing so reads to this USAian to be outside of typical Canadian character. I could be wrong.
These days those transmitters are deprecated in favor of satellite hops. (Outside of littoral waters hiding a submarine under water is close to impossible these days. Satellite photography reveals their locations with wonderful huge arrows painted on the water surface from their underwater wakes. The Navy REALLY does not like to talk about that, for fairly easy to understand reasons.)
Note that at least for a while NIST maintained a time standards broadcast on 20kHz-26kHz FSK, WWVL. That service has terminated with WWVB at 60KHz being the VLF service today. So the USN clearly does not have a lock on everything below 500kHz. It may seem that way. People have mostly forgotten that those frequencies can be used with exceptional ranges if you devote enough wire to the antenna. For an extreme antenna example check out "Jim Creek". For a smaller example check out WWVB's antenna.
For sonar - there is no way the USN could snarf up everything below 500kHz. There is no international allocation body to allow that. And it would effectively turn off two and a quarter bazillion fishing sonars, bottom obstruction avoidance sonars, and all the other private sonar uses.
(I've babbled too much already - I'll be quiet for awhile unless specifically addressed.)