4. I seriously doubt any "tourism" dollars would justify it.
It seems to me the prevalent use would be for Windsor residents working in Detroit, of whom there are plenty. The success of such a ferry would hinge on connecting shuttle service to the DMC, WSU, Ford Hospital, and other employment "magnets." Nurses and college personnel (staff and students) are two of the target demographics, I'd guess. (Yes, I dated a nurse living in Windsor and working in Detroit once upon a time.) The cost of the ferry and shuttle would have to be competitive with tunnel/bridge fees and parking/gas. The problem, as I see it, is that there's no symmetry ... Detroit residents working in Windsor.
"Tourism" just doesn't hack it. I can't envision the ferry being an automobile ferry (a la Seattle) since the tunnel and bridges (Ambassador and Port Huron) are quite adequate other than during rush hours. A passenger ferry service for "tourists" would be ridiculous, since neither Detroit nor Windsor have a pedestrian-friendly ecology ... either in geographic compactness or weather. Thus, neither are pedestrian "destinations" like NYC, Chicago, or San Francisco ... it's about as silly to think of pedestrian tourism in Detroit/Windsor as it would be in Los Angeles.