No, no, can't be done. When you get onto I75, you leave Sault Ste. Marie and enter state property, and stay on that until you reach the Ontario border, which then runs directly into Sault Ste. Marie, Ontario. From within the city limits of Sault Ste. Marie, MI, there is no access to the St. Mary's River rapids, unless you are willing to break the law and walk across either the flood gates, or the train tracks.
And so, as you can see, RJD, as stated, there is no access from Sault Ste. Marie, MI.
Seeeeeeeya; Chief Longwind of the North