What kind of cooler should I get for a boat?

Get a rotomolded cooler like Yeti, Pelican, or RTIC. They hold ice for two to three days in direct sun and heat because the thick plastic and foam insulation are molded together with no air gaps. Prioritize a model with a drain plug and tie-down loops. You don’t need the most expensive brand — RTIC makes nearly identical coolers for less.