[Kakkerlak]

Hold your breath as you read this. Keep holding it until you feel that urge to exhale. That's your body reacting to the concentration of CO2 in your lungs.

That's one of the characteristic dangers of CO2, because it causes you to start breathing rapidly to attempt to expel it, which only increases the concentration. Your heart rate goes up, you burn more oxygen, and its a quick spiral into unconsciousness and asphyxiation and death.

Most industrial asphyxiations happen in sealed environments like boilers and sumps, that don't have open tops or ventilation ports. But I sure wouldn't take the risk on any boat.

Definitely do not attempt to use dry ice for cooling on a boat. Good old water ice is safe, plentiful, and a lot more fun to drop down your crew's shirts.