When you checked with the police, I'm surprised they didn't come to comfiscate the kayak. With a couple of the cop shops I'm familiar with, it would then become a new toy for someone in property management, or if it was a piece of junk it would be in a police auction.
This is not meant to be a slam against law enforcement, but just what I've experienced.
Just a word of warning: If, or when the owner is located, don't be too put off if you don't even get a thank you. I've returned things that I have found and got no more than a suspicious look from the owner. Anyway, I applaud you for doing the right thing; you are obviously an honest person and that is its own reward.