But as ship as big as that one should probably have something even larger. A couple of single barrel 20mm cannon would surely discourage the snot right out of 'em. They were common secondary and anti-aircraft armament on torpedo boats, submarines, and other light vessels. Heck something that big could mount a 4 or 5 inch deck gun. Liberty mounted at least one, and more often two, a 3 inch forward and 5 inch aft, along with 10 or so 20 mm cannon, and they were much smaller than a modern cruise ship.
My uncle was Naval Armed Guard on the SS William Clark, Torpedoed and sunk by U.354 in Greenland Sea at approx.71.05N 13.10E on 4 Nov 42.)
Uncle Lennie survived the War, but some of his toes did not. (Frostbite, it's dang cold in the Greenland sea in November.)