Oh, I already sent the PM. Guess I'll just copy/paste it here:
Okay, so my problem is with the very ending. The drowning. So it makes sense that that's the point where that version of Booker took the baptism and became Comstock, but why is that necessary? He accepts the Elizabeths drowning him, meaning that "our" version of Booker has control—if he has control in that moment, why not just refuse the baptism? Comstock would never be created. Hell, if he's both past Booker and present Booker, why not just refuse the baptism (destroying every possibility of Comstock in the future) and live happily ever after from that moment on with Elizabeth?
The whole drowning bit seems completely unnecessary when he could just avoid making the decision in the first place and continue living.
And what I forgot to add in the PM is that it seems that it would even be a better ending, because he becomes a better person throughout the events of the game. Sure, not becoming Comstock and thus having no one to sell Anna to will keep him from being as bad of a person as the Booker in the beginning was because of her presence, but he still did a lot of things that he regretted by that point (which seemed pretty clear from the bit at the Hall of Heroes) and would still be the kind of person who would sell her in the first place. In a sense, he just allowed for a version of himself where he avoided that one mistake as opposed to continuing on as an all-around better person.