My understanding is that a ship's crew in international waters can defend itself however it deems fit. I don't know about restrictions by ports regarding armed ships, though I suspect any port along or near the Indian Ocean would be rather understanding.

Personally, I think a good .50-caliber deck gun (or two) that could be moved from one mount to another would be enough to discourage most would-be Rambos. A pair of trained marshalls, taking turns at watch, should be sufficient to repel most attacks.

Defenses would not have to be 100% effective; even a 90% chance of getting shot to pieces would kill the pirate industry. There are safer ways to earn a dishonest buck.