I'd encourage all of you who bring coolers to stick a thermometer in it. Just because there's a gallon jug with ice in it doesn't mean everything inside your (my god those things are expensive!) name brand whatever is 32, 42, 52 ... degrees. Try a thermometer, I'm sure it will surprise you as much as it always did me.

Keeping things cold is something I cannot consistently do to my satisfaction even after all these years. Evaporative cooling is helpful on windy days - the lower the dew point the better. Cool is doable depending.... You can keep fillets overnight in most any temp but they will still taste, ummm, like seafood in the midwest. Cheesecake? Pudding? No problem.

I've tried a wide variety of approaches and what I miss most is aluminum canoes in Spring. I've even intended to do the math and experiment on whether or not the same size bottle filled with either gas or water (ice) or whatever makes a difference.

Your mileage may vary - GREATLY!     It all depends on what you want or need to keep cool or cold and for how long and how you view risk.

I remember one group trip where we picked out the moldiest (green as I recall) pieces of Kraft pre-shredded cheese ... based mostly on how much we needed to sufficiently cover the pizza before the other toppings went on. Use by dates are meaningless up there and I doubt anyone of us washed our hands before but I know I did after picking out moldy cheese ... excellent pizza none the less.