Carbon monoxide has no smell. On boats it comes from engines, generators, heaters and other boats' exhausts, and it collects in cabins where people sleep. A-24 is the standard for the detectors that warn them.
What it covers
ABYC lists A-24 as Installation of Carbon Monoxide Detectors and Alarms, and published its own summary of the latest revision. ABYC says A-24 requires carbon monoxide detectors to monitor the atmosphere in habitable spaces: spaces with provision for sleeping, cooking, eating, navigation or steering, and washing or toilet use. Storage lockers, open cockpits and engine rooms are not habitable and typically don't need detectors.
What surveyors check against it
Walk each habitable space and look for a detector. Read the label: ABYC says only detectors tested by an independent laboratory to UL 2034 should be installed. Check where it's mounted: not where it can be damaged, not beside hatches, ports or vents where the air is diluted, and not in dead corners. Press the test button. Check the replacement date; ABYC's guidance is to replace units before it.
Then the sources: a generator, a diesel heater, a gas cooker, and the engine exhaust. See no CO alarm and exhaust elbow corrosion.
Citing it in a finding
"Fit CO detectors tested to UL 2034 in the aft cabin, forward cabin and saloon, ABYC A-24." List the habitable spaces and which have a detector.