From Adam Clarke's commentary:
1 Peter 3:19
By which - Spirit, his own Divine energy and authority.
He went and preached - By the ministry of Noah, one hundred and twenty years.
Unto the spirits in prison - The inhabitants of the antediluvian world, who, having been disobedient, and convicted of the most flagrant transgressions against God, were sentenced by his just law to destruction. But their punishment was delayed to see if they would repent; and the long-suffering of God waited one hundred and twenty years, which were granted to them for this purpose; during which time, as criminals tried and convicted, they are represented as being in prison - detained under the arrest of Divine justice, which waited either for their repentance or the expiration of the respite, that the punishment pronounced might be inflicted.
This I have long believed to be the sense of this difficult passage, and no other that I have seen is so consistent with the whole scope of the place. That the Spirit of God did strive with, convict, and reprove the antediluvians, is evident from
Genesis 6:3; : My Spirit shall not always strive with man, forasmuch as he is flesh; yet his days shall be one hundred and twenty years.
And it was by this Spirit that Noah became a preacher of righteousness, and condemned that ungodly world,
Hebrews 11:7, who would not believe till wrath - Divine punishment, came upon them to the uttermost. The word πνευμασι, spirits, is supposed to render this view of the subject improbable, because this must mean disembodied spirits; but this certainly does not follow, for the spirits of just men made perfect,
Hebrews 12:23, certainly means righteous men, and men still in the Church militant; and the Father of spirits,
Hebrews 12:9, means men still in the body; and the God of the spirits of all flesh,
Numbers 16:22;
Numbers 27:16, means men not in a disembodied state.