That is not completely correct.
There are thousands of commercial fishing vessels up to 5000 GRT without COIs that can and do, write a 12 hours a day seatime letter for every day onboard regardless of hours worked or whether the vessel was underway, at anchor, in port., or hauled out for a couple of weeks.
Years ago commercial fishing boats had no logbooks and few records. Now they have fantastic fishing logs that are routinely reported to NMFS, a federal agency, and therefore become official government records (same as the discharges).
10 years ago, I proved some of my factory trawler seatime from the 1990s with my copy of the fishing logs sent to NMFS. The evaluators wouldn’t accept it, but it was accepted on Request for Reconsideration.
I think very few people, and only the rarest of companies , would paw through old logbooks trying to figure out who did what for how long on everyday aboard. Thats impractical, expensive, and rare.
If anything, seatime from the majority of recreational boats is overly optimistic and very generous.
Seatime letters from 99% of companies is based upon the payroll. If you were on the boat’s payroll, you must have been working.
The only companies that would spend time and money scrutinizing your seatime to write a letter , are the ones who have be caught and prosecuted for persistently writing fraudulent seatime letters. The only company I can think of like that was Tyson Seafoods over 30 years ago.
I vaguely recall that some of the OSV companies had similar problems long ago, and that a couple of USCG officers at REC New Orleans were convicted of help people cheat.
If there is a COI for a vessel, and it mandates three watches, the USCG is not supposed to allow 12 hour days. But one never knows what some evaluator may do.