This way they conduct it as a "play", Booker plays Comstock and is killed in his place. In other words Booker is drowned which is then used as the universal ending, for any universe where Booker accepts the baptism as he cannot be drowned by refusing the baptism.

How is Booker DeWitt alive in burial at sea?

Infinite's final twist saw this version of DeWitt die too - drowned by a multiverse of Elizabeths to pre-emptively stop all versions of Comstock from ever existing. ... With the timeline reset, this DeWitt was back in his old life - possibly with his infant daughter, too - in a reality cleansed of Comstock's interference.

What happened to Elizabeth after BioShock Infinite?

The Luteces begrudgingly come to Elizabeth's aid and explain that when she died, she remained in existence because of her quantum-superposition, but she could not return to Rapture without collapsing her unique quantum state and becoming a normal person - a result of returning to the dimension in which she first died.