Maybe this verse (topic) deserves a thread on it's own. Maybe it's just one of those never ending trinity/free will debate.
As a summary: The preaching was done during His life. The prisoners are Israel. (captives of the law?)
First, it seems very unlikely that Peter is saying that Jesus was "made alive" before the making alive of the resurrection. It is much more likely that "having been made alive by the Spirit" refers to the resurrection (cf. Romans 1:3-4). The verb zōopoieō is used most often in the New Testament to speak of the actual resurrection of believers (John 5:21; Rom. 4:17; 8:11; 1 Cor. 15:22, 36).
The verb poreuomai, which is used for "having gone" in verse 19, is used in verse 22 (and in Acts 1:10-11) for the ascension. That would suggest the following sequence: death, resurrection, ascension, proclamation to the spirits in prison. It would still be puzzling (there is nothing to correlate it with), but it would not require an un-Jewish "intermediate state" between death and resurrection.
However, Peter (the apostle) says that in the same Spirit by which he was raised from the dead Jesus also went to "proclaim to the spirits in prison". If we take poreutheis as a reference not to the ascension but simply to Jesus' ministry to Israel "in the Spirit", we would then only have to understand the allusion to Noah figuratively. Peter draws an analogy between the world before the destruction of the flood, when God's patience (makrothumia) with sin had been exhausted, from which a few escaped by the ark, and "unrighteous" Israel before the judgment and destruction of AD 70, from which a few were saved by the antitype of baptism.
So Jesus did not go in the spirit and proclaim to the generation of humanity before the flood. He went in the power of the Spirit that would later raise him to life to proclaim to a generation of Jews which was like the generation of humanity before the flood.
"In prison" (en phulakēi) would be a metaphor for Israel's captivity to sin or satan (cf. Matt. 12:29; Mk. 3:27; Lk. 11:21). Isaiah says that the servant of the Lord will "bring out from bonds those who are bound and from the prison (phulakas) house those who sit in darkness" (Is. 42:7 LXX).
It is basically the same argument that Jesus makes in Matthew 24:37-38 (= Lk. 17:26-27): just as the world was taken by surprise by the judgment of the flood, from which only Noah and his family were saved, so Israel will be taken by surprise by the judgment of the coming war, from which only a few will be saved. Paul upbraids the Jews for presuming on God's patience (makrothumia), which was meant to lead them to repentance (Rom. 2:4); and he asserts in Romans 9:22 that God's patience (makrothumia) has run out with Israel.http://www.apologeticspress.org/APContent.aspx?category=10&article=851
In 1 Peter 3:18-20, a most curious reference appears on the surface to be an affirmation that Jesus descended into the spirit realm and preached to deceased people. However, a close consideration of the grammar will clarify the passage. First, the preaching referred to was not done by Jesus in His own person. The text says Jesus did the preaching through the Holy Spirit: "…the Spirit, by whom…" (v. 18-19). ["My Spirit" (Genesis 6:3) = the Spirit of God = the Spirit of Christ (Romans 8:9; Ephesians 2:17).] Other passages confirm that Jesus was said to do things that He actually did through the instrumentality of others (John 4:1-2; Ephesians 2:17). Nathan charged King David: "You have killed Uriah the Hittite with the sword" (2 Samuel 12:9), when, in fact, David had ordered it done by another. Elijah accused Ahab of killing Naboth, using the words, "Have you murdered and also taken possession?" (1 Kings 21:19), even though his wife, Jezebel, arranged for two other men to accomplish the evil action. Paul said Jesus preached peace to the Gentiles (Ephesians 2:17), when, in fact, Jesus did so through others, since He, Himself, already had returned to heaven when the first Gentiles heard the Gospel (Acts 15:7). So the Bible frequently refers to someone doing something that he, in fact, did through the agency of another person.
In fact, within the book of 1 Peter itself, Peter already had made reference to the fact that the Spirit "testified beforehand the sufferings of Christ and the glories that would follow" (1 Peter 1:11). But it was the prophets who did the actual speaking (vs. 10). Then, again in chapter 4, Peter stated that "the gospel was preached also to those who are dead" (1 Peter 4:6). Here were individuals who had the Gospel preached to them while they were alive ("in the flesh"), and who responded favorably by becoming Christians. But then they were "judged according to men in the flesh," i.e., they were treated harshly and condemned to martyrdom by their contemporaries. At the time Peter was writing, they were "dead," i.e., deceased and departed from the Earth. But Peter said they "live according to God in the spirit," i.e., they were alive and well in spirit form in the hadean realm in God's good graces.