Are Booker and Comstock the same person?
Comstock is an alternate version of Booker DeWitt. After the Battle of Wounded Knee, Booker was overcome with guilt for the atrocities he committed and sought a way to absolve himself of his sins. ... Elizabeth reveals that to truly destroy Comstock, in the set of realities where he accepts the baptism, Booker must die.
Why is Comstock in burial at sea?
The Comstock in BaS escaped the destruction of the Columbia multiverse because the Luteces had already sent him to the Rapture multiverse, so he wasn't affected by his original timeline being erased.