I've read a few articles about placing ice in the livewells. They mention although the fish will keep better and stay lively in the cooler water, that the critical point for their survival is when they are taken from that nice cooled down oxygenated livewell, and put back in the 90 degree lake water. It's a shock to their system.

Kind of like walking out of your 68 degree air conditioned house into 95 degree 80% weather outside, the bigger the temp difference the bigger the shock to your system.