So, I was thinking about this while driving to the post office and back. It seems to me that the hardest part of defending a large vessel with a small crew is the inability to detect them before boarding. If the ships don’t show up on radar, the crew needs some other warning system, with a proximity alarm that gives them time to demand hail friend or foe, or blow them out of the water (or off the side of the ship if they’re already climbing). Assuming that they are willing to arm themselves (it doesn’t really take that much to take out one of these pirate boats give sufficient warning), and it’s legal, the only missing element would be the warning system.
So how hard and expensive would it be with today’s technology to rig cameras around the ship with motion detectors, and software to filter out waves? It seems like a pretty easy problem to me. I’d think that most modern digital cameras are smart enough. Give them IR capability, and they’d work through fog.