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"p.s. A friend of mine had an incident years ago when his CO2 alarm system was faulty and he almost died" ARE YOU SURE ABOUT THAT?
It is extremely unlikely that your friend has or had a CO2 detector. Carbon Dioxide is very different from carbon MONoxide, which is what a "CO" detector picks up. CO, not CO2.
(If you'd had a decent cup of coffee this morning you'd have realized that.)
CO is toxic, because it binds to the hemoglobin in the blood--and binds very well--so there is no place for the blood to transport oxygen, and then you die. Cigarette smokers lose something like 5-10% of their hemoglobin capacity (blood oxygen levels) simply from lighting up and smoking, by the way. If you take someone out in the air, give them oxygen, it makes no difference after CO poisoning, because their blood cannot absorb the oxygen and they will either die or recover regardless.
CO2 is NOT TOXIC. It displaces oxygen, and is an INERT gas. Remove someone from a CO2 atmosphere, and they'll gasp and wheeze and start breathing and recovering immediately. If you are someplace where there is a lot of CO2, you'll motice it, and respond to it, and even wake up from it, giving you a chance to get out. Unlike CO, which makes you sleepy and keeps you asleep until you die.
Very very different gasses.
Back to the original question: A brick of dry ice in the icebox shouldn't be a problem, as long as there is normal ventilation in the boat and you're not sleeping in the bilges or on the floors with a huge chunk of it. I'd close off the icebox drain hose while using it--since the cold gas is still cold enough (I think) to keep regular ice frozen, and keep things cool. Any warm gas that comes out the top of the box, should mingle with the air and disspate without bothering anyone.
And of course, let everyone on board know, if they "feel funny" to let the captain know and get on deck at once--just in case.