Certain ICOM HF sets, most notably the original (and classic) M700, have a logic operated squelch that is based on voice recognition. Believe it or not, this function actually works, and it makes maintaining an HF watch on board actually tolerable.
In theory, mayday broadcasts in the marine world on HF would be sent on 2182 USB simplex. In real life, there is almost no one listening on 2182, and in most locales, the CG has equal or better range on 16FM. Most of the captains I know would use 6215 USB simplex if it was hitting the fan, hoping to reach one of the CAMSLANT receivers.
Such things as GMDSS, sat phones, and SelCall have all but eliminated most routine marine HF traffic. There are still a few marine nets around (usually calling on 6215 UHF simplex and shifting to one of the other ITC 6 MHz simplex channels for traffic). One thing that most of us used to listen to regularly was the daily net for offshore yachts maintained by Herb Hilgenberg (a/k/a "Southbound II"). However, Herb moved from 8 MHz to 12 MHz after Bermuda kicked him out of Hamilton, and I don't know for a fact that he has continued his much valued (and much monitored) service.
Edit: I gather Herb is still around:
South Bound II VAX498 - Ship routing and weather forecasting
