Funny thing, the most effective defensive weapon so far has been firefighting equipment
I guess a well-aimed hose has about the same range and better accuracy than an RPG fired from a dinghy.
Personally, I figure they should put 3-man Seal teams on a random selection of cargo vessels, armed with a .50 and some portable anti-armor (TOW equivalent) weapons. Invite the pirates in a bit, pop open a hatch and kablooi.
And the best part - don't publicize any of it, especially if they actually blow up some pirates. Pirates simply disappearing without a trace will be a pretty good deterrent. And any captain on any boat in international waters has the authority to execute pirates, so it's even more legal than shooting an armed intruder in your own house (which you can't do in many countries, but those countries recognize the ultimate authority of a captain on the high seas).