Snide disapproval at politics encroaching touring aside, where specifically are you going? AFAIK, the ferries are as follows:
Ludington to Manitowac
Muskegon to Milwaukee
Blue Water Ferry between Michigan and Ontario near Sarnia/Port Huron
Mackinac Island (probably your best choice, especially if you have to cross the bridge)
Drummond Island
Beaver Island
Bois Blanc Island
I think there is one around the Les Chaneux Area, not positive tho
Sugar Island near Sault Ste Marie, but that is all of three minutes across