I keep a cooler in my truck in which I keep 2 to 3 frozen large elongated spring water bottles. Right before I launch, I put the water bottles into the back hatch on a bed of burlap bags.
Once I catch some fish, I wrap them in the burlap bags,with the ice bottles. Depending on the water/air temp, the ice usually last for 3+ hours.
Ice cold water on a hot day is a added bonus