There are no vampires, werewolves, gods or demi-gods in the novel. I realize this might seem strange, given that immortals have become a requirement in any story for young readers, but I simply couldn't see a way to fit them in. If I were to add a werewolf to the crew, he would inevitably transform during the crossing, since it typically took a sailing ship a month or more to go from one side of the Atlantic to the other in the age of piracy. This werewolf would probably kill all the other pirates, and then, when he woke up, all exhausted and guilty-feeling, but certainly not hungry any longer, he wouldn't have anyone to help him sail the boat.
OK, so what about vampires? There is one character who could absolutely fit the role - any guesses? - but the pirates and vampires mash-up has already been done.
Gods? Certainly Poseidon could have a role in any ocean-soaked book, but I didn't see any place for him here.
Instead, Fish is populated solely by mortals. Real people who won't survive the shot from a pistol or the thrust of a cutlass.