[Boasun]

If you have dry ice inside the boat, you better have excellent ventilation.
Dry Ice does not suck out the oxygen, it displaces the air. For it is heavier than air. Thus your oxygen has been displaced. For this reason I never allowed dry ice taken inside the vessels that I have worked on. I know that it is extreme caution but then all of my crews are still alive. I don't take chances with with my crew's safety un-necessarily.
But a couple of coleman coolers, with dry ice, lashed topside would work. Then you could time how long it lasts. Also all of the frozen stuff transferred from those coolers will act as ice block in the ice chest down below.

Two key phrases here: Coleman Coolers & Lashed topsides.