Racing toward you here at something over 104 mph and throwing a watery white tornado into the sky with her prop is Miss Supertest III, Canada's entry in the Harmsworth Trophy, emblem of international motorboat racing supremacy. Somewhere back of that " tornado is the American boat Maverick that was supposed to make the 12th consecutive successful U.S. defense of the coveted cup. Maverick didn't quite make it. Miss Supertest III, driven by Robert Hayward, a Canadian chicken farmer, and owned by J. Gordon Thompson and Jim Thompson of London, Ont., broke the U.S. grip on the trophy for the first time since 1920 when famed Gar Wood won it from England. Said Gar (now 78), "We held it too long."