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You are confusing charters with liveaboards, particularly the multi-day ones in warmer waters. As far as I've been able to find, most liveaboards require booking and payment in advance, although maybe some might allow a small number of last minute walk-ons who wouldn't have paid until then. And I haven't found any where you can get your money back for last-minute operator cancellations, although the better operators reportedly at least try to put you to a decent resort on land.
You aren't looking very hard.