Note entirely correct.
Lusitaina was configured with mounts for 12 6” guns.
U-Boats of the time were surface vessels with the limited ability to submerge. Most merchant sinkings were by the deck-gun as it was more dependable than the torps of the time...
The guns, however, were never mounted, and well before 1915 the Lusitania was taken off the Auxiliary Cruiser list, mainly due to her high rate of fuel consumption.