1 Peter 3:18-20
18 For Christ also suffered once for sins, the righteous for the unrighteous, that he might bring us to God, being put to death in the flesh but made alive in the spirit,
19 in which he went and proclaimed to the spirits in prison,
20 because they formerly did not obey, when God's patience waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight persons, were brought safely through water.
First neither hades nor the grave are ever called "prison" and "prison" is never called hades or the grave anywhere in the NT.
Jesus' earthly ministry is given in Luke
Luke 4:17-19
17 And the scroll of the prophet Isaiah was given to him. He unrolled the scroll and found the place where it was written,
18 "The Spirit of the Lord is upon me, because he has anointed me to proclaim good news to the poor. He has sent me to proclaim liberty to the captives* and recovering of sight to the blind, to set at liberty those who are oppressed,
19 to proclaim the year of the Lord's favor."
This mission did not include preaching to the dead either in the grave or hades. The word translated" captives" means "prisoners of war."
If the proclaiming 1 Peter 3:19 was to redeem those in prison then it was a failure only 8 were saved and they were alive when they were saved not dead.