I would contact the Magazine websites directly. Sail, Cruising World, Sailing World, and some of the lesser known sailing magazines have web pages. At the very least you should be able to get thier annual publications statement that is required by the US federal government and which appears annually in the back of all major US magazines. The bigger magazines track their own demographics and publish them from time to time. You might also contact US Sailing who also maintains a lot of demographic records.