Gar Wood built busses in Marysville boat factory from 1936 to August 1939 when the division was sold to General American Transportation Co. The boat would most likely have been produced in those production years. There are no Production records of this model being produced in Marysville during that time.
Forever the optimist the boat does resemble the Streamline Cabin Utility produced from 1939 to 1941 but there are enough subtle differences in the window styling that make me skeptical (see Gar Wood Boats by Anthony Mollica pg92).
Also of note, Gar Wood buses were framed and welded at a Gar Wood plant in Detroit and only skinned in Marysville so I’m not sure the boat factory would have been equipped with a jig to construct a steel frame boat. To definitely rule out the possibility one would need to see production records from the Detroit plant.
All that said, its one sweet boat that looks a whole lot like a Gar Wood to me.