I've often wondered the same thing...that, or they have a pre-determined net/time or frequency/time schedule set up. Turn the SSB radio on channel X at this time, listen on channel Y or frequency Z at that time, etc. Or, they simply leave their radios on whichever frequency/frequencies they use and call each other whenever they want....but since its the HF marine band, there's always the possibility of another user on that same frequency.
If they're not using selective calling of some kind they either only turn their radios on at certain times or have very good squelch circuits!
https://www.navcen.uscg.gov/doc/rtchan.txtAngry Bahstads - ITU 6 MHz SSB Marine Channel 605
Lobstermen with Broken Fridges - ITU 6 MHz SSB Marine Channel 608
Both groups (on 6212 kHz USB and 6221 kHz USB) are using duplex channels for simplex purposes. Ship transmit frequency, of course. I wonder if the radios come pre-programmed with all the channels in duplex mode and simplex mode.