Boat U.S. advertises 3rd party liability insurance only on their site. covers damage to others and clean up costs of sinking of your vessel, though no hull value. Here's their description lifted from their site:Liability PLUS Policy – Provides coverage for third-party property damage or bodily injury. PLUS valuable extras not found in a standard liability policy. Excludes coverage for your boat and equipment.
site is:http://www.boatus.com/insurance/policy.asp/