HF-SSB marine bands - these are standardized maritime mobile MF and HF communications bands - note that many maritime mobile users operate outside of these bands and there are dozens of country- or region-specific additional bands/frequencies. These bands are used for USB voice transmissions, various data modes including FAX used for Maritime Fax Transmissions, FSK, modes for various purposes as well as USB voice and RTTY weather and emergency broadcasts to ships at sea.
Fisheries, fishing boats, and other commercial marine users will often use frequencies in the fixed/mobile bands (including 43 meters - 6765-7000 kHz) clear frequencies in the shortwave broadcast bands or even amateur frequencies for "out of band" communications to reduce the chance of radio interception by their competition. Out of band frequencies are also used in other ITU regions for marine fax transmissions.
See also: VHF marine band
MF-SSB and HF-SSB marine traffic (usually in the bands above 4 MHz) can be in simplex or semi-duplex for voice and/or data. Cross-band operation is also allowed per ITU regulations. Often you will hear ship-to-ship communications in simplex mode on the "ship transmit" frequency of a duplex pair.
MF SSB and HF SSB marine bands
|405-525 kHz||MF band, used in some areas for NAVTEX on 490 kHz and/or 518 kHz. 500 kHz (600 meters) was the original CW Morse code calling frequency|
|1600-2000 kHz||MF band, used in Europe and elsewhere (also used heavily for fishnet beacons) - overlaps with 1800-2000 kHz 160 meter amateur band|
|2000-2300 kHz||MF 2 MHz marine band - USB voice calling 2182 kHz, DSC calling 2187.5 kHz|
|2450-2850 kHz||MF 2 MHz marine band|
|4000-4063 kHz||4 MHz fixed/mobile band, shared with marine for simplex purposes|
|4063-4438 kHz||HF 4 MHz marine band - USB voice calling 4125 kHz, DSC calling 4207.5 kHz|
|6200-6525 kHz||HF 6 MHz marine band - USB voice calling 6215 kHz, DSC calling 6312 kHz|
|8000-8195 kHz||8 MHz fixed/mobile band, shared with marine for simplex purposes|
|8195-8815 kHz||HF 8 MHz marine band - USB voice calling 8291 kHz, DSC calling 8414.5 kHz|
|12330-13200 kHz||HF 12 MHz marine band - USB voice calling 12290 kHz, DSC calling 12577 kHz|
|16460-17360 kHz||HF 16 MHz/17 MHz marine band - USB voice calling 16420 kHz, DSC calling 16804.5 kHz|
|22000-22855 kHz||HF 22 MHz marine band|
|25070-25121 kHz||HF 25 MHz marine band|
|26145-26175 kHz||HF 26 MHz marine band|
|26175-28000 kHz||Various CB-like marine allocations found in this region|