Carbon monoxide is not just a risk to boaters. Swimmers, too, can be overcome by breathing in CO and drown in just minutes!
Carbon monoxide can build up near exhaust vents from inboard engines, outboard engines, and generators. Areas of high risk are under the back deck, swim platforms, or in between the pontoons on houseboats. Do not swim in these areas. Wait at least 15 minutes from when the motor or generator has been shut off before entering these areas.
There have been known cases of drowning attributed to carbon monoxide where swimmers have come up for air at the point of an exhaust pipe on a vessel.