What are red flags when inspecting a used boat?

Major red flags on a used boat: soft or spongy deck areas (water-damaged core), heavy bilge staining from persistent leaks, mismatched gelcoat repairs indicating past impact damage, HIN that does not match the title, engine that is hard to start or smokes heavily under load, excessive hours without documented service history, freshwater seals that are cracked or brittle, and a seller who refuses a sea trial or independent inspection.