Chapter 15?
"As already noted, baptism is not a requirement for salvation in the first place ( Ephesians 2:8; Romans 3:28; 4:3; 6:3–4). "
1 Corinthians 15 is a challenging chapter to interpret. Yet, with other scriptures, we can see Paul is not saying proxy water baptism will save those who are dead. The chapter is about the resurrection of Christ and the saints.
Paul may have been addressing a pagan practice. Or there are other interpretations many have proposed.
I am not going to try and tell you or anyone what I think, but I will stay with the context of the chapter and speak on the Resurrection of the believer. And the eyewitnesses who saw Jesus after he was resurrected.