I always assume most of these are fishermen, by the content and context of the conversations, but some are cryptic. Talking about the 6000 khz - 7000 khz stuff. There are some persistently used frequencies, like 6900 khz, can be USB or LSB that seem to be more or less 'freebanders' as they never give out callsigns.
There are many others and one way to snag them is to monitor Fleetnet and Safetynet ground stations on 1539 MHz on Inmarsat. This is fairly easy with a small homebrew antenna, a RTL-SDR and LNA.
Use Scytale-C (free) or the pay decoders, like inmarsatdecoder or Tekmanoid. Scytale-C is actually pretty good now, and decodes almost as well as inmarsatdecoder.
You will often see marine stations requesting their buddies go to certain HF frequencies all up and down the bands, from 4000 kHz to 11000 kHz.
I use an antenna made of cardboard, aluminum foil and copper tape. Works great. I have a printout you can use as a form to build the helical coil, if interested.