Verbal Kint(aka Keyser Soze?!) narrates 'the story' of what happened after their police lineup, few weeks back, that ultimately leads up to the finale at the ship. How far is the story told by Kint to David Kujan is true ?.
It's kind of the point and charm of the film that your narrator is unreliable. All we can know for sure is that a boat was blown up, all of it's passengers were killed and that they were trading people when Keyser Soze was killing many men. Other than that, no other points of the story were qualified as true. There weren't even police reports for the New York's Finest Taxi Service heist. This is a great question. I would even venture to say that it is THE question. But it is totally unsolvable, I'm afraid.