You are talking about engaging a paramilitary force with a high likelihood that they have combat experience. All you will do by being armed is get yourself and your passengers and crew killed and wounded, and increase the number and types of weapons that the pirates have. Your most effective defense would be to make yourself appear to be of little to no value to them. Don't travel the dangerous parts of your trip with the boat decked to the nines, and everybody on board dressed like they have a bank account. Unless you've go armor, don't engage, an RPG is no joke (been shot at with more than a few), and anyone who thinks they can take on a well armed pack of third world killers with commercially available weapons is both foolish, and suicidal. Unarmed, you will almost certainly make it through alive, if a bit behind schedule, and lighter in the wallet. Your first shot would be about all you'd get, and it would be promptly answered by automatic rifle fire, rockets, and whatever else they have to throw at you. Forget sniping in open water, too, unless you've done it before (and allot), you won't hit anything but sky and ocean.