You know that the crews are not only discouraged, but usually not allowed to fight back with real firearms for fear of retaliation ? The pirates are ready to die, after all. The most they'll do is use a fire lance to keep the pirates away or wash them off-board.
Cruise ships (and some other ships) generally have armed gaurds. I heard that it is popular among Israelis (after conscription and before working) to work on a cruise ship as armed security.
(really all I want to do is go fight pirates)