In the comments over at gcaptain and Eagle Speak there is some suggestions to arm ship's crews to defend against pirates. Capt Kelly Sweeny makes a good argument at Professional Mariner.
Any proposals to arm crews would have to include plans to train crews in the use of firearms. The problem of declining standards of seamanship skills and overworked crews seems to have been overlooked.
Does it make sense to spend time and money training merchant crews in the use of lethal force? The reluctance to arm crews is not because of any squeamishness or some kind of misguided political correctness but practicality. I get ABs from time to time that can not be taught to steer. At safety meetings we still are trying to get the concept of wearing eye protection when using power tools across to the crew, with limited success. Where is the time to train and supervise crew armed with automatic weapons going to come from?
In practice arming ship's crew across the spectrum of shipping would be very difficult. Ship's crews will always be responsibly in large part for on board security but in areas where the threat is high, the Gulf of Aden for example, ships will require shore-side assistance. The required extra security will have to come from either to riding crews, naval forces or both.
At Marinelink is an article that discusses real world problems with arming merchant crews.