Interesting in "Go Boating" magazine.

brother & sister while sitting on swim platform of stationary boat while motor running at idle surcom to carbon monoxide poisoning while father was putting on ski vest. Girl was laying in prone position on platform and became unconscious.
Brother wassitting on platform as he was removed from the platform he complained of severe headache; vomited and passed out. Both sent to hospital and survived.

A study done on recreatioal boats by the institute for occupational safety &health did an exstensive study on carbon monoxide.
The stern of a boat proved to be the most dangeous place to be regarless regardless of whether it was a ski boat; houseboat; cuddy or full cabin

The study also showed that all of the boats with high carbon monoxide were some sort of four stroke, wether inboard or stern drive.The old two stroke was the worst with carbon emmissions.
The two stroke DFI had hardly any carbon monoxide registering at any point in the boat.