Uhh.... Hello... he has already made the trip and reported back - inspite of the Dry Ice!!!!W.Star, every boat is different, every icebox is different. Bear in mind that it can take 24 hours for the box to cool down, so you want to load it up with ice (whichever kind) a day or two in advance. If possible, stock it up at least 24 hours in advance and then add more ice before casting off.
I've never heard someone complain "we had too much ice" but I have thrown out spoiled food from not having enough.
Dry ice is great stuff, if you have a small igloo cooler or even a white styrofoam cooler you can always take some out if the box has gotten too cold. Or wrap it in towels or insulation to tone it down. And as has been mentioned--make sure it doesn't ventilate into the cabin, if the cabin is not well vented to being with. Unlike CO2, CO won't just up and kill you in your sleep, it will give you enough discomforts to get you up and out of there--but you still don't need that bother if you can avoid it. And of course, again unlike CO, once you get to fresh air you're out of trouble, there is no CO2 buildup in the blood to block oxygen getting back in.