I glassed in a tag under both my hatches with all the pertinent information, which was more for proof of ownership when crossing borders and recovery identification. I also purchased an insurance policy which cost $130/year (Canadian$) for $15'000 of coverage, which I believe to be worth it (fire, theft, lost at sea, collision while on the car etc).
I would suggest a tag just to prove to unbelievers that you actually built it yourself

James

: Calling on all my kayak-theft experience, I would have to say, "No, an
: owner plaque would not deter me from stealing a kayak." On the other
: hand, a hidden ID might help in identifying a recovered kayak.

: Mike