CO poisoning and CO2 asphyxiation can occur anytime there is an unvented appliance in use in a closed space. That is why there are laws and regulations that prohibit unvented heaters in residences.
Safe use of an unvented appliance (Many are in use in construction projects every winter.)requires a fair amount of ventilation through the walls of the structure to maintain oxygen supply by free communication with outside air.
The cooking stove in a boat is an unvented appliance, whether it is fuelled by alcohol, kerosene, gasoline, LPG, or wood. These flames are small and intermittent, and boats are, generally, quite well ventilated, but it is still good practice to keep some vents open just in case.