Did he spray foam below the waterline or only above? If he sprayed below, don't buy it. The bilge will become smelly and you'll never be able to clean it. Makes no sense to insulate below waterline except to hide something.
you can always reach in under the flooring and cabinetry with a camera to take photos in areas of plating you can't easily see by eye. It really pays to take a good look yourself. Most surveyors won't go to the trouble and they have no means to gauge thinning out of metal.