Magazines are entertaining but really are primarily there to provide marine vendors an avenue to sell things to you--their next target--the new boater. If you think you'll end up with an older and smaller fiberglass boat (less than 35 ft), by all means pick up Good Old Boat magazine. You can learn alot about the various boats out there in the used market, modifications, how people are using and maintaining them. Nice publication. Ocean Navigator used to be pretty nice but I note it also seems to be getting crammed with ads and is thin in content.
Spend an afternoon every other week at the local Borders Bookstore, browse and read and figure out what you like. Eventually you have it figured out if you wish to purchase any of the popular press.
Depending on where you are, you can find free reading about sailing and cruising. Here on the West Coast of the US, we have Latitude 38, and the Log which are about area boating but throw in some cruising info as well.