I have no idea of the international laws but it would seem since the Achille Lauro incident that it would be prudent to have some type of military or police protection on board.
I would think maybe a few marines with M-14's and maybe one larger gun would be enough.
Nice pic above to help understand the typical ship boarding pirate attack, on a ship of this size.
My guess is the crew got lucky and spotted the pirates early, and the captain took radical evasive movements, preventing the pirates from getting their grapnel hooks and ladders aboard.