I use one of the freezer totes Tom Powers mentioned. A ten pound bag of ice works well. I'll throw a wet towel over it on really hot, sunny days. I fish from a Hobie Revo and keep the cooler bag between my seat back and my crate. I put the ice in a trash bag in the tote so I can keep my catch and my lunch/dinner and water separated. The bags seem to last 2, maybe 3, seasons at most. The bag is the silver thing behind my back:
Anadromous fish come and go, but catfish stay forever.
Yellow Hobie Revolution
Yellow Tarpon 120