Hakkoda-maru is a preserved Seikan ferry, one of the ships that carried trains, passengers, and freight between Aomori and Hakodate before the Seikan Tunnel changed the route north. It sits by the harbour, close to Aomori Station and the Bay Bridge.
The ship is most interesting if you like transport history, but it also explains Aomori's geography. For decades this city was not the end of Honshu so much as the place where mainland Japan turned into a sea crossing.