What's the best way to clean a fishy cooler?

Use baking soda and vinegar. Rinse the cooler, scrub with a baking soda paste, rinse again, then wipe with white vinegar. Let it air dry with the lid open. Baking soda neutralizes odors, vinegar kills bacteria. Don’t mix them at the same time. Bleach is overkill and can leave a chemical smell.