1 Peter 3:18-21 interpretation by avoiding assumptions

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, 
Peter has been talking at length about the importance of suffering righteously under government, under husbands, and what our attitude and mindset should be while suffering. He holds up Christ as our model, who when reviled did not open His mouth; when suffered, He blessed.  Suffering for a purpose is the larger context of the letter at this point, which we must keep in mind when we sail into the more choppy waters of verse 19.
I propose that in examining this verse we avoid making undue assumptions, and select the meaning that doesn't cause an avalanche of assumptions that lead away from Peters larger context.

in which He went and proclaimed to the spirits in prison, 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.
Assumption: the verse speaks of a chronology.  That is, after dying Christ then went to preach to the Spirits in prison. 
However, this very natural assumption does not hold up, because logically the text would go Noah, Crucifixion, preaching. While there is every reason to think that Christ went to sheol after death considering He didn't go to the Father (John 20:17), and that He went to preach His triumph and lead a captive host free, there are really no grounds to assume this is what Peter had in mind given his discussion on suffering.  Once made, this assumption too easily joins hands with the next one

Assumption: the Spirits were in prison during the time of preaching.

It must be true if Christ was preaching after His death, but since that one is dubious, this one is doubly so, and worse, it's dragging a third assumption with it

Assumption: the Bible is using prison not as an explanation but as a place
which is unsound given the way we as humans speak, and how often the Bible engages in commentary.  For example, I might ask you "When did you first meet your wife?" and you would understand my question, even though I'm importing a later relationship that doesn't "currently" exist, as a reference.  Technically I'm committing an error in calling her your wife before she's actually your wife, but it's likely you would not attach significance to this, you would assume my meaning and tell me of the time the two of you first met, before you were married, although strictly speaking you first met your wife at the alter. 
The Bible often references relationships like this, I'll just throw out one I just read to prove the point, 1 Kings 21:25-26.  The more natural way to read this is as the NASB says "proclaimed to the Spirits who are now in prison"

Assumption: spirit means angels, or considering they were in prison, fallen angels.
This one tends to unhinge any remaining interpretation completely, because it must be asked, what are they in prison for? And the answer must be a stupefyingly huge assumption that they were the sons of God who had kids by the daughters of men, (which is itself really two assumptions: demons can take on bodies as desired, and demons are genetically and anatomically compatible with women to produce hybrid offspring.) Which is frankly, totally absurd. 


Without those assumptions here is how the text reads
in the same way He died in the body and was alive by spirit which He went and proclaimed to the spirits of humans who are now in prison, and who are there 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.  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

The idea is that during the ministry of Noah, Christ's spirit was patiently waiting, suffering at the abuse of wretched men. He was preaching to them to repent, to live, but they refused to obey and so all perished underwater.  Likewise, Christ suffered under their abuse on the cross, and his body perished, but His spirit was saved.  Likewise too, just as the ancient world was wiped out with water and righteous Noah was spared you have had your sinful nature annihilated by the flood so that your spirit could be saved. Your flesh has died with Christ in baptism, and your spirit has been raised to a new life.
So Christian do not be alarmed that you suffer in the flesh, or that it is wasting away, but be of good cheer, your spirit will be all the more alive for it. (See 2 Cor 4:16)
And no need to muddle through all sorts of weird theology to get there either.