KTFLC is the ultimate rule of Raft Cooler Management (RCM). RCM is an obscure yet evolved science, and the rule stands for Keep The F’ing Lid Closed, often yelled loudly from camp kitchen to beer-getter. Here is a summary of the science to date: air and water = melting, so eliminate both from the cooler.
Line the bottom with block ice—it stays frozen longer than cubes (although you’ll need some of those for mixed drinks). Drain the coolers when the ice starts to melt. Freeze food in advance and pack the coolers tightly. Duct tape an inventory to the lid so items are easy to find (remember KTFLC), and pack smart so coolers are emptied top-down, one at a time.
Keep the lids strapped down at all times, and lay a wet wool blanket or reflective sheet on top during day hikes so they don’t heat up in the sun. Leave a cheap thermometer inside each cooler to ensure it stays below 7°C/45°F. If it gets warmer than this, redistribute the important items to other coolers and scarf the rest. A bout of food poisoning swiftly dampens the communal enjoyment of rafting. Certificates and diplomas are available in RCM. Contact your local degree-granting institution for details.