The 216 foot long, 959 dwt passenger ship Yorktown went aground on the Detroit River between Wyandotte, Michigan and Fighting Island, Ontario.   The Yorktown departed Detroit with 120 passengers bound for Cleveland, Montreal and Quebec City.  There was no damage and the passengers were allowed to remain on board.   A tugboat was requested to help refloat the vessel.    The Coast Guard dispatched investigators to the scene to conduct a survey of the vessel.   Reports state the area where the Yorktown grounded was known to be quite shallow.