If you're going to pass through a reasonable amount of Wisconsin these maps will be invaluable. I've used them for the last 15 years. The set of four is well worth the price (~$16-20).
http://www.abtrails.com/item487.htm
BTW, I'm in Manitowoc where the ferry crosses to Ludington, MI. It's about a 4-hour boat ride.
http://www.ssbadger.com/home.aspx
There's another that's faster (~2.5 hours) that's out of Milwaukee if that helps:
http://www.lake-express.com/