Interesting. Have you noted any identifiers of any kind when hearing these guys? I've only listened to them a handful of times and no callsigns have been heard. FWIW, 6212 kHz is ITU 6 MHz marine channel 605. It's listed as a duplex frequency (for ship-to-shore anyway, paired with 6513 kHz...with 6212.0 kHz being the "ship transmit" frequency). With that in mind, it makes sense for them to use it as a simplex frequency for ship-to-ship comms. Apparently this is pretty common practice. Since their SSB radios likely came pre-programmed with all the ITU marine frequencies out of the box, that would also make sense.
My bet is 6212 USB is their official or home channel, and the various other 6 MHz frequencies we've monitored them on are side channels, "secret channels", alternate frequencies, etc. etc. As I mentioned in the other thread, during my recent visit to Gloucester, MA, I did notice many of the fishery buildings had HF antennas on their roofs. Maybe 6212 is their "company channel" and the other frequencies are used so the boss-man and/or other ships in their fleet can't listen in?
As an aside, the majority of ITU marine frequencies (channelized HF marine band plans for 2 MHz, 4 MHz, 6 MHz, 8 MHz, 12 MHz, 16/17 MHz, 18/19 MHz, 22 MHz and 25/26 MHz) involve duplex pairs. Only the following frequencies are explicitly designated for simplex ship-to-ship or ship-to-shore use - specifically SSB voice:
2 MHz (2000 kHz - 2850 kHz) marine band:
2003.0 kHz
2065.0 kHz
2079.0 kHz
2082.5 kHz
2086.0 kHz
2093.0 kHz
2096.5 kHz
2142.0 kHz
2203.0 kHz
2214.0 kHz
2635.0 kHz
2638.0 kHz
2738.0 kHz
2782.0 kHz
2830.0 kHz
4 MHz (4000 kHz - 4438 kHz) marine band:
4146.0 kHz
4149.0 kHz
4417.0 kHz
4000 kHz to 4060 kHz are also designed for simplex maritime radio use, in 3 kHz steps (4000 kHz, 4003 kHz, 4006 kHz, etc.), shared with fixed/mobile service
6 MHz (6200 kHz - 6525 kHz) marine band:
6224.0 kHz
6227.0 kHz
6230.0 kHz
6516.0 kHz
8 MHz (8000 kHz - 8815 kHz) marine band:
8294.0 kHz
8297.0 kHz
8101 kHz to 8191 kHz are also designed for simplex maritime radio use, in 3 kHz steps (8101 kHz, 8104 kHz, 8107 kHz, etc.), shared with fixed/mobile service
12 MHz / 13 MHz (12330 kHz - 13200 kHz) marine band:
12353.0 kHz
12356.0 kHz
12359.0 kHz
12362.0 kHz
12365.0 kHz
16 MHz / 17 MHz (16460 kHz - 17360 kHz) marine band:
16528.0 kHz
16531.0 kHz
16534.0 kHz
16537.0 kHz
16540.0 kHz
16543.0 kHz
16546.0 kHz
18 MHz (18780 kHz - 18900 kHz) marine band:
18825.0 kHz
18828.0 kHz
18831.0 kHz
18834.0 kHz
18837.0 kHz
18840.0 kHz
18843.0 kHz
22 MHz (22000 kHz - 22855 kHz) marine band:
22159.0 kHz
22162.0 kHz
22165.0 kHz
22168.0 kHz
22171.0 kHz
22174.0 kHz
22177.0 kHz
25 MHz (25070 kHz - 25121 kHz)
25100.0 kHz
25103.0 kHz
25106.0 kHz
25109.0 kHz
25112.0 kHz
25115.0 kHz
25118.0 kHz
I put together the list above as these frequencies are also likely to be pre-programmed in MF / HF SSB marine radios, in addition to the dual-use duplex/simplex channeling (3 kHz steps for SSB voice, except odd steps on 2 MHz band). It should also be noted that many older-generation SSB marine radios do not cover the higher bands. Many radios only cover up to 12 MHz / 13 MHz or 16 MHz / 17 MHz.