I found neither my house insurance or car insurance would adequately cover all my boats. My house insurance company (Maine Mutual Fire Insurance) however wrote me a separate policy to cover my several canoes, kayaks, and dingies against loss. They based the premium for each boat on the opinion of value I provided. Some coverage for accessories and for liability is also provided. It's expensive though, adding up to more than my house insurance, so I may drop coverage for at least some of the boats. Depends on what you comfortable with. A friend of mind lost a boat off his truck racks last year on the New Jersey Turnpike and heard a big truck behind run over it; he kept driving, there being no point in going back. Fortunately the boat was insured.
-Tom