Merchant Marine Sailors are not trained to be War Fighters. What about having a Private Security Team which is sufficiently armed and trained to watch for fend off the Pirates. They could be billeted seperately from the crew and rotate watches. In case of an impending attack they would engage the attackers either from lowered speed boats or from the ship. I am sure company’s such as Blackwater or The Gryphon Group and others would be happy to bid their services to ship owners. I am sure it would be a lot cheaper than what is happening now with Ransom and detained vessels.
Neither are the pirates. I'm sure American sailors have "some" exposure to weapons, though foreigners probably don't.