Thank you...
...Japan was sadly ineffectual, thank goodness.
However, the German HSK raiders were remarkably effective and could have caused more havoc. Minefields outside Halifax, NS and New York, NY, could have been disastrous for the convoys. So, too, could clandestine overflights by seaplanes to report when the convoys were due to depart. An HSK could have been used to land spies, saboteurs and frogmen, to attack storehouses and moored or anchored ships. The Great War saw German agents set off a massive explosion on a quayside, so they have the experience to do the same in the Second War - a more effective Operation Pastorius.