Most of the question marks around buying a used boat can be removed by hiring a professional marine surveyor to do a pre-purchase survey. The surveyor is paid by you, the Buyer, and the surveyor serves only in the your best interests. This is your primary protection and is, in a sense, the cheapest insurance available.
Although the great majority of owners are honestly presenting their boats, there may exist serious deficiencies that are not apparent except to a professional marine surveyor. Many structural problems are not visible and are determined by the surveyor using a variety of low tech and high tech methods. Even boats that are just a few years old could have major problems. You’ll not only learn of any deficiencies, but you’ll also be provided with the surveyor’s estimate of the boat’s Fair Market Value (FMV).
Knowing the boat’s FMV will help in price negotiations and in getting boat financing. When you apply for marine insurance, your insurance company could require that a survey be done. That may be after-the-fact when you actually own the boat. It’s better to know the boat’s actual condition and value before you buy it.
In hiring a marine surveyor, you should be aware that the most consistently competent surveyors are members of either NAMS (National Association of Marine Surveyors) http://nams-cms.org/ or SAMS (the Society of Accredited Marine Surveyors) http://www.marinesurvey.org/. Their web sites will direct you to surveyors in your area.