KnowX has a Watercraft Ownership database that covers vessels that are (a) over 37 feet long and owned by U.S. citizens or U.S. corporations or (b) involved in "coastwise or fishing trades" (and hence registered with the Coast Guard).
Lexis has a database (P-PROP;BOATS) of all ships and boats registered with the Coast Guard and/or over 30 states. Lexis says, "Most documents contain the following: owner's name and mailing address, registration number, date, and county, title number, decal number, hull identification number, vessel make, model, year, and length, vessel propulsion and fuel type, vessel use information."
The N.A.D.A. publishes several Guides for figuring the value of ships, boats and other watercraft. The N.A.D.A. Web site lists the full line of N.A.D.A Guides and also lets you look up values for most ships, boats, etc. sold since 1984.
Accidents: The Coast Guard Maritime Information eXchange includes a database of Incident Investigation Reports including all "reportable marine casualties investigated by the U.S. Coast Guard from November 2002 to present."
Stolen Boats: Generally boat buyers have to call the police if they want to know if the hull number, registration number or other number on a boat or boat part has been reported as stolen. However, Florida has posted an online search so boat buyers can check Florida State Police records quickly and easily on their own.
Foreign Vessels: Lloyd's Register of Ships, published annually, lists registered vessels from all countries. So do the Fairplay Maritime Directory Annuals.