Flapbreaker,
Never a bad idea and relatively cheap insurance. California thinks enough of the problem to begin requiring CO Warning stickers at the helm and on the transom, mainly because of accidental deaths of people playing around the stern when moored in areas with very little natural ventilation, like very small coves and grottoes at the lakes. Not necessarily from the engines, but often from generator exhaust.
A friend of mine installed one in his V-berth just for safety and the silly thing kept going off. He thought it was a malfunction and replaced it. The replacement went off. He discovered an exhaust leak in his gen in the engine compartment that was allowing CO to move forward to the berth.
Since all of us use outboards, the chance of an engine generated source is small. However, the Wallace Stove or one of the portable generators some are using could be a source.
For divers, even small amounts of CO is dangerous as it adheres to red blood cells and reduces exchange of both nitrogen and oxygen. I'm taking extra precautions to eliminate any chance of picking up CO at my compressor intake.
And I'm definitely installing a sensor/alarm forward.
Scaley