Why is carbon monoxide a potential danger on a boat, particularly one with an enclosed cabin or exhaust near swim platforms?

1. Carbon monoxide is harmless and only affects engine performance
2. Carbon monoxide is an odourless, colourless gas that can accumulate in enclosed or poorly ventilated areas and cause serious harm or death
3. Carbon monoxide only affects vessels with sails
4. Carbon monoxide is only a risk if the vessel is moving at high speed