I have only ever seen HINs. Engine serial numbers are useless. To answer your theoretical question as to what would stop a thief from jacking your boat and "just" swapping hulls is, it would be a major operation. Just pulling an engine is a laborious task in itself. They would be better off just chopping the thing for parts. Further more, the HIN is usually located at multiple points in the hull, usually fore and aft. I have seen them under the battery box on some Kawis and on the front of the hull and sometimes on the tunnel. It will vary with manufacturer, but as far as theft goes, it's not really economically feasible for a thief to steal a boat, then swap the internals into a hull with clean numbers and try to resell it. Cost of clean hull + time to disassemble/reassemble + look for someone to sell it to at a used price = Not worth it.