A boat fire spreads fast and there's nowhere to go. The extinguisher by the companionway is the crew's first and often only chance, and the year stamped on its bottom decides whether it counts.
What the rule says
In US waters, the Coast Guard's rule for recreational boats now sits in 33 CFR 175 Subpart E. In its own bulletin: a portable 5-B or 20-B extinguisher must be taken out of service on 31 December of its 12th year. Disposable dry chemical units carry the year of manufacture as two or four digits, normally stamped on the bottom. Rechargeable units, such as CO2, are checked by an inspection tag from an NFPA 10 technician instead.
New and replacement units must be marked Marine Type USCG. Extinguishers must be readily accessible, and good and serviceable: gauge in range, pin in place, nozzle clear, no significant corrosion or damage. Monthly inspections and records are no longer required of recreational boaters. On boats of model year 2017 or older, B-I and B-II units without a date stamp can stay in use if they're serviceable.
Outside the US, the flag, the insurer or a charter licence sets the rule. The checks a surveyor makes are the same.
How a surveyor checks them
Count them and note where they are: engine space, galley, helm, cabins. For each one, turn it over and read the year, then check the gauge, pin, nozzle and body. Look at how it's mounted and whether it can be reached quickly. A unit buried under lines at the bottom of a locker isn't readily accessible.
Then fit the finding to the boat. No serviceable extinguisher at all, or none near the engine or the galley, is an A. Expired units alongside good ones are a B.