(Spoilers) Grandfather Paradox and Multiverse Memory.
Gradfather Paradox?
The Gradfather paradox basically states that if you traveled back in time and killed your parent (or grand parent) before you were conceived you were not to be born. If Elizabeth was not born, then how is she able to drown Booker at the baptism?
Also, how is it that Booker has memories at the Baptism when he is drowned there? Did he suddenly retain memories of his multiverse self at that moment prior to drowning? And if that is the case, then the drowning wouldn't be necessary because if All instances of him drowning resulting in no Comstocks, then all instances of the baptism could have been prevented without him dying.