Coolers keep things cool longer. I can see the salty ice water absorbing heat from the fish faster (thus slightly warming the salty ice water faster than non-salty ice water, but it doesn’t make sense that the contents of the cooler would take less time to reach room temperature. If you start off with a cooler at -15 degrees, it will take longer to reach room temperature than a cooler at 0 degrees; If not, then you don’t have a insulated chest, you have a bucket of ice.