From what I understand, you cannot book the tour until you're on the ship. Most people that want to do the tour will sign up for it FIRST THING. They get on the ship, and before they even get up to the Lido, before exploring, they head straight to guest services and sign up for it.
I think there are only two or three opportunities for the week, so it does fill up really quickly.
Random third-party websites are no substitute for personal experience.
Pirates: Putting the Fun in Funville since 2011