I have not donated kayaks but have been asked to do so. I have donated lesser value things that I've made.

One thing you should be aware of is that you can only take a tax deduction on the cost of your materials, not on the "fair market value" of the kayak. i.e. if you spent $500 and spent 100 hours building and your time is "worth" $50/hr you can only deduct $500, not $5,500. So, from the tax perspective it is not a great deal, but it can be a nice way to support a cause you feel is worthwhile.

http://www.freelancetaxation.com/can-i-deduct-my-art-when-i-contribute-it