I'm not an expert on the topic, but I'm pretty sure ships cannot burn bunker fuel unless in international waters. They have to switch to diesel when this close to the shore.
That being said, the calculation that a cruise ship generating its own power emits as much SO2 as 34400 idling trucks seems to hold up.
According to the article one cruise ship idling emits as much pollution as several myriad semi trucks idling. That seems significant to me and moreover easy to abate.
Also note that semi trucks aren't allowed to idle in NYC.