The after credits ending just shows that the whole thing was a dream to convince Booker not to give away Anna. Comstock represents himself being a horrible father for giving her up, along with other characters representing as Booker's other aspects. That would also explain why Elizabeth looks like a Disney Princess (when everyone else look somewhat realistic). The drowning represents Anna never forgiving her father for giving her up, and the whole multiple doorway represents that Booker has a choice. Along with many others.