We carry a large cooler for food & perishables.
We put in 2, frozen, one gallon water bottles, and 6 or 7. frozen, individual serving water bottles.
If there is any room leftover, we top it off with a bag of ice, or preferably a block of ice. That keeps things cold for the best part of 4 full days. There will still be some ice in the gallon water bottles when we get home. As ice in smaller bottles melts down, we drink the water.
Whether in base camp, or river camping we keep a wet towel over the top of the cooler.
The fewer times you open the cooler; the longer the ice will last.
For that reason, if we’re base camping, we have a separate cooler for soda, juice, milk, and beer. Makes for fewer openings of food/perishables cooler.