1 Peter 3:19-20

¹⁹ by whom also He went and preached to the spirits in prison,

²⁰ who formerly were disobedient, when once the Divine longsuffering waited in the days of Noah, while the ark was being prepared, in which a few, that is, eight souls, were saved through water.

by whom (v.19) — referring back to “spirit” in verse 18 (which see for various interpretations of what spirit is meant.

Wuest has a long, involved explanation of how he reached his conclusion about what verse 19 means. I’ll just quote his conclusion.

Our Lord, between His death on the Cross and His resurrection from Joseph’s tomb, preached to the fallen angels in Tartarus [hell]. But what did He preach to them? The word translated “preached” here is kerusso. The word was used in secular Greek of an official announcement or proclamation made by a representative of a government.  The word itself does not indicate the content of the message. — Wuest, page 100.

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The sin of these angels was committed just previous to the flood. We have established the fact by a study of the usage of the word pneuma, translated in 3:19 “spirits,” that Peter is referring here to angels. He states that they were disobedient at the time of the flood. … These angels have been cast down to hell. … [Based on Jude 1:6-7] the sin is … fornication. … This fornication was in character the “going after strange flesh” [Jude 1:7]. The word “strange” is the translation of the Greek word heteros which means “another of a different kind.” In committing this sin of fornication, the angels transgressed the limits of their own kind and invaded the realm of another order of being. The sin of Sodom was the transgressing of the male beyond the limitations imposed by God (Romans 1:27). … In the case of angels, the forbidden flesh (lit. “other than appointed by God”) refers to the intercourse with women, in the case of Sodom, to the departure from the natural “use” (Romans 1:27). It was a departure from the appointed course of nature and seeking after that which is unnatural, to other flesh than that appointed by God for the fulfillment of natural desire.”…

These fallen angels and their sin of committing fornication with women of the human race is spoken of also in Genesis 6:1-4 in the words “The sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.”…

[In regards to why angels took human wives.] It would carry with it a confounding of two distinct orders of creatures and the production of a mixed race, partly human, partly super-human, which wold be just such a derangement of the divine plan as to warrant that which occurred, namely, the almost total extermination of all who were upon the earth. Now, connecting this with the fact that our Lord after His death on the Cross, went to Tartarus and made a proclamation to these fallen angels, we suggest that the probably purpose o f the angelic apostasy so far as Satan was concerned was the derangement of the divine plan of the incarnation and substitutionary atonement of the Son of God, for if his purpose had succeeded, God would not have incarnated Himself in a race part angel and part man. The last Adam was to be God the Son come in a human incarnation to answer in His humanity to the humanity of the first Adam. The action of God in completely exterminating the race and saving Noah and his family prevented the spread of this unlawful mingling of angelic and human natures, and allowed the incarnation to take place. The proclamation was probably to the effect that, in the incarnation and the Cross, God had defeated the scheme of Satan to defeat His purpose. it would therefore be a proclamation of victory. — Wuest, pages 101-105

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Peter speaks of the ark, “wherein few, that is, eight souls were saved by water.” the words “were saved” are literally in the Greek, “were brought safely through.” The preposition “by is from dia, a preposition of intermediate agency. That is, the souls in the ark were brought safely through the time of the flood by the intermediate agency of water. While it was true that it was the ark that saved them, yet Peter is not teaching that here. He says the waters of the flood saved them. They buoyed up the ark above their own death-dealing powers and saved those inside the ark. The very waters that were death to the rest of the human race were life to the inmates of the ark. — Wuest, page 107.

Stam agrees with Wuest entirely. Williams has another view. I think Wuest and Stam have it right.

The sense of the passage therefore is, that during a hundred and twenty years while the Ark was being built, the Spirit of God in Noah warned men of the coming flood. Only eight believed; the rest disbelieved, and, as a consequence, were shut up in the prison of the abyss. The believers were sheltered in the refuge of the Ark, and were saved through the waters of the judgment in the baptism of the Ark. — Williams, page 1002.