You can listen to some of that range for yourself for free:http://www.eham.net/ehamforum/smf/index.php/topic,87234.0.html
DC to 500 KHz covers a ton of stuff:
<15 KHz earth sound things
11-59 KHz submarine digital communications. Digital but often really loud.
60 KHz WWVB (digital time/frequency)
160-200 European Longwave AM broadcast... can be heard in most of the U.S. sometimes
285-325 KHZ Differential GPS
200-529 KHz Nondirectional air beacons (NDBs)
472-478 Experimentally licensed hams on various modes. CW, PSK others
506-7 KHz More ham
518 Navtex ship weather beacons
530 Highway Info stations
I don't know of any two way communication between ships still being done in the 500 KHz range. Almost everything is now digital. Hams have been known to use SSB on longwave but very rarely.