Never brought dry ice into canoe country but I have a nifty story about the kindness of Canadians.
We were coming back from Sioux Lookout Ontario and a Lake Minnitaki fish camp we used to frequent for a week every summer. We had our coolers filled with frozen fish (compliments of the proprietors of the camp) and were determined to get some dry ice. I stopped in at an ice cream store in Thunder Bay and inquired.
The owner knew of a welding shop on the other side of town that had some. He started to describe how to get there and then abruptly said to "just follow me". With that we were off and I assumed it would be a hop, skip and a jump from his store but 15 minutes later and on the other side of town we arrived. I was shocked, how very kind ;-)
A little info. I think dry ice works best with already frozen items. Keeping them frozen longer while you get deeper into the bush. It will also help keep regular ice frozen as well. Dry ice is CO2 so it will attract mosquitoes as well. If there is any left place it away from your camp to attract them over there.