This is a very simple explanation. And if we follow Ockham's razor, the simpler explanation should prevail. The whole movie is a dream. DoCaprio's character is a man whose wife suicided. He hasn’t told his kids yet, and cannot face them as he feels guilty about it. His father in law (Caine) takes care of them. He is haunted by the wife’s death and wished there were a form of psychotherapy so he could return to a more normal life. He is on a business trip and flies back home. In the plane, he falls asleep (maybe after watching David Lynch’s Mulholland Drive?), and ... starts dreaming. The whole movie is his dream. All the passengers around him are assigned a role in the dream, including his father in law. Except Ariadne who is the exact equivalent of DiCaprio in Titanic, an angel that disappears at the end. Both are not in the passengers list, both guide the hero and save him, both dive at the end. Ariadne is Jack, DiCaprio is Rose in a role inversion.