The firehoses are good defenses, from what I'm told, against the less sophistocated pirate vessels.
Better yet, hook the firehose up to a fuel tank, and have another crewman standing by with a flare launcher...
Oh, that's a GOOD and evil idea, actually. When in pirate waters, keep a high-pressure pump on deck that can handle solvents. Bring up a drum of diesel, bunker oil or whatever. Spray at marauding vessel, start launching flares.
I always wondered about that-AK's are great, and so are RPGs, but against a vessel? That's like hunting polar bears with a beeman.
All they need to hit is the bridge, even the base of the funnel to screw up the ship pretty badly. Also, most merchant ships do not have very thick hulls. An antitank-penetrator RPG into the stern would likely punch right through the shell plating and explode in the engineering spaces.
For other antipirate things in the future...one word. Railgun.