In Colorado, boating laws require that the owner put his name and address inside the boat, even for kayaks. The last thing I want is a thief to know my address, so I left that off but did Magic Marker in my name and manufacturer serial # (for which I have the original receipt) on my present mass-produced kayak.

For the S&G that I am now building, the kit maker provided ID papers and advises glassing them onto the boat. As others have said, this is not really a theft deterrent; the numbers are small and often placed in hidden areas. And a thief could always sand off the info anyway. I am going to apply the ID numbers but ALSO will put something unique and obvious on the exterior of the kayak. This could be as simple as a name/graphic -- just something unique. People sometimes paint their commuter bicycles wild color schemes to make it stand out. Same principle. It won't guarantee it from being stolen but makes it a riskier target than a "looks like lots of others" kayak.