The (American) Union Jack

The Union Jack.

According to Joseph McMillan at SeaFlags, "Under current Navy regulations, an ambassador's presence in a boat of the Navy within the waters of the country to which he is accredited, is signified by flying the union jack at the bow staff, topped with a spread eagle finial. In 1946, the State Department provided for the same flag's display on automobiles carrying chiefs of diplomatic missions, including ambassadors."