Thank you for your interpretation.
Unfortunately, I don't see anything here about baptism for the dead. I think these verses make much more sense when we read the entirety of the chapter, and it is always helpful to go to other translations as well, just in case the KJV is too...archaic, as it can be at times (I'm not saying it is with these verses).
Firstly, Peter is speaking to living people in these verses. Before verse 18, we see that he is telling the people that even if they suffer for righteousness' sake, they will be blessed, and that it is better to suffer for good than to suffer for doing evil. We then go to verse 18, where we are told that Christ suffered for our sins, so that He might bring us to God. He also went to the spirits in prison (verse 19). Who are these spirits in prison (at least those being referred to in this case)? They are those that did not obey during the time of Noah's ark (verse 20). Now, we must remember the story of Noah's ark. In that story,
8 people survived the flood (Noah, his wife, his 3 sons, and their wives, totaling 8). They were saved by, or through the water, and they are the 8 being referenced in verse 20. Verse 21 then goes on to find similarity between Noah's ark and baptism. Where in Noah's ark 8 people were saved by/through water, so now we are saved by/through the waters of baptism. The "us" referenced in verse 21 is referring to those being spoken to, not those in prison.
I think it's pretty clear from reading these verses in context that this is the correct, and standard, interpretation of them. The "eight" are the eight that were saved in Noah's ark, and they are not the one's being talked about as being in prison. Those in prison are those that were disobedient during the time of Noah's ark.
Catholics in the United States use the New American Bible (at least in liturgical settings), and I will quote from it below, which I think makes the reality of what the verses are talking about even more clear. I'll start at verse 13:
"13Now who is going to harm you if you are enthusiastic for what is good?
14But even if you should suffer because of righteousness, blessed are you. Do not be afraid or terrified with fear of them,
15but sanctify Christ as Lord in your hearts. Always be ready to give an explanation to anyone who asks you for a reason for your hope,f
16but do it with gentleness and reverence, keeping your conscience clear, so that, when you are maligned, those who defame your good conduct in Christ may themselves be put to shame.
17For it is better to suffer for doing good, if that be the will of God, than for doing evil.
18For Christ also suffered* for sins once, the righteous for the sake of the unrighteous, that he might lead you to God. Put to death in the flesh, he was brought to life in the spirit.g
19In it he also went to preach to the spirits in prison,*
20who had once been disobedient while God patiently waited in the days of Noah during the building of the ark, in which a few persons, eight in all, were saved through water.h
21This prefigured baptism, which saves you now. It is not a removal of dirt from the body but an appeal to God* for a clear conscience, through the resurrection of Jesus Christ,i
22who has gone into heaven and is at the right hand of God, with angels, authorities, and powers subject to him.j
The commentary from the NAB for these verses also supports my interpretation:
* [3:1322] This exposition, centering on 1 Pt 3:17, runs as follows: by his suffering and death Christ the righteous one saved the unrighteous (1 Pt 3:18); by his resurrection he received new life in the spirit, which he communicates to believers through the baptismal bath that cleanses their consciences from sin. As Noahs family was saved through water, so Christians are saved through the waters of baptism (1 Pt 3:1922). Hence they need not share the fear of sinners; they should rather rejoice in suffering because of their hope in Christ. Thus their innocence disappoints their accusers (1 Pt 3:1316; cf. Mt 10:28; Rom 8:3539).
We find further support of this in the Catechism of the Catholic Church:
1219 The Church has seen in Noah's ark a prefiguring of salvation by Baptism, for by it "a few, that is, eight persons, were saved through water":
The waters of the great flood
you made a sign of the waters of Baptism,
that make an end of sin and a new beginning of goodness.
1094 It is on this harmony of the two Testaments that the Paschal catechesis of the Lord is built, and then, that of the Apostles and the Fathers of the Church. This catechesis unveils what lay hidden under the letter of the Old Testament: the mystery of Christ. It is called "typological" because it reveals the newness of Christ on the basis of the "figures" (types) which announce him in the deeds, words, and symbols of the first covenant. By this re-reading in the Spirit of Truth, starting from Christ, the figures are unveiled. Thus the flood and Noah's ark prefigured salvation by Baptism, as did the cloud and the crossing of the Red Sea. Water from the rock was the figure of the spiritual gifts of Christ, and manna in the desert prefigured the Eucharist, "the true bread from heaven."
Finally, I will quote from the ESV, which I believe is a translation popular amongst Evangelicals, and is a literal translation as well (I believe the KJV is also a literal translation):
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. 21 Baptism, which corresponds to this, now saves you, not as a removal of dirt from the body but as an appeal to God for a good conscience, through the resurrection of Jesus Christ, 22 who has gone into heaven and is at the right hand of God, with angels, authorities, and powers having been subjected to him.
Hope that helps.