I've had a theory about that particular verse and the one preceding it.
Because of the totality of lives lost (other than the 8 souls saved in the ark) I believe that these souls, Noah, his wife, his 3 sons and their wives, were the "prisoners in prison" in verse 1 Peter 3:19.
I discussed this theory online with a pastor who completely disagreed with me. No surprise there. He had other ideas about trapped angels in prison. People think angels are all these powerful ministering spirits in heaven. But, not all. Some of them are quite ordinary people, sometimes disobedient, imprisoned by their sins, yet as we know of Noah, preached righteousness. He was not perfect, but he definitely qualifies as an angel. He and his family definitely qualify (description wise) as prisoners in prison.
But, 1 Peter 3:19 is a puzzling verse that I think is clarified in the following verse which identifies that only 8 souls were saved. To me, that identifies the prisoners in prison to be Noah and his whole family.
It makes perfect sense that the online pastor disagreed with you, because you are incorrect.
The grammatical structure of 1 Peter 3:19-20 makes it perfectly clear that Noah and his family are
not the spirits in prison. Here are the verses from the NIV and NASB to illustrate (with my notes in square brackets):
NIV: After being made alive, he [Jesus] went and made proclamation to the imprisoned spirits— 20 to those who were disobedient long ago
when God waited patiently in the days of Noah while the ark was being built. In it [the ark] only a few people, eight in all, were saved through water
NASB: in which He [Jesus} also went and made proclamation to the spirits in prison, 20 who once were disobedient
when the patience of God kept waiting in the days of Noah, during the construction of the ark, in which a few, that is, eight persons, were brought safely through
the water.
In both versions, the "when" indicates a distinction between the clause before and the clause after. The rules of grammar simply don't allow your conclusion to be valid.