I always thought of pirates as loud swaggering singers. But the thought of pirate chants has intriguing aspects. I guess depending on where you are from would determine whether a pirate uses a Byzantium or Gregorian mode. I think I would enjoy a Phrygian mode chant because it could have a jazz feel to it.
I had more of a drunken wobble and carousing sing song lilt to it while my marsupial chrge had a somewhat frazzled hiss tint to him.