No, I was still digging.
In 2016 the USCG addopted NFPA 10 portable fire extinguisher requirements by reference. They expressly removed the record keeping requirements in 2021 for recreational boats (in 2016 they were focused on commercial boats and the application to recreational boats was overlooked), but intensionally left the rest. You really can't make an argument against a sign.
NFPA 10.
13.6.3.1.3.10.1. Cabinets housing fire extinguishers shall not be locked, except where fire extinguishers are subject to malicious use and cabinets include a means of
emergency access. [10:6.1.3.10.1]
13.6.3.1.3.10.2. The location of fire extinguishers as described in
13.6.3.1.3.3.2 shall be marked conspicuously. [10:6.1.3.10.2]
So yes, it is a requirement to label extinguishers in cabinets. The rule is not easy to trace, and many do not follow it, but I have seen many labeled, and the rest seem to be just ... wrong. I spoke to a local USCG guy, and he said yes, they write warnings for extinguishers in un
marked cabinets.
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An annoying thing about many standards is that they are copyrighted, and so you cannot link to them. I do regret that.