1Peter 3:18-20 says
 For Christ also hath once suffered for sins, the just for the unjust, that he might bring us to God, being put to death in the flesh, but quickened by the Spirit:
 By which also he went and preached unto the spirits in prison;
 Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
(1Pt. 3:18-20 KJV)
Are these the dead that have the goodnews proclaimed to them in 1Pt. 4:6:
For for this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit.
(1Pt. 4:6 KJV)
Or are they the watchers of 2Pt. 2:4-5 = Jude 1:6 (and 1Enoch).
 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
 And spared not the old world, but saved Noah the eighth person, a preacher of righteousness, bringing in the flood upon the world of the ungodly;
(2Peter 2:4-5 KJV)
And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
(Jude 1:6 KJV)
1Pt. 3:19 seems to parallel 1Pt. 4:6
1Pt. 3:19-20 also seems to parallel 2Pt. 2:4-6 = Jude 1:6 (and 1Enoch)
Historically, the misunderstanding of what both 1 Pet 3 & 4 are saying, along with the conflation of these with 2 Peter & Jude (not to mention the disregard of Enoch)., have led to the Christian mythology and consequent theology of the "harrowing of hell" by Christ (see the Apocalypse of Peter et. al.). All of the above listed passages in the question are related to the imprisoned celestial beings talked about in Enoch, the only exception being the 1 Pet 4 passage. Looking at the verses before the sixth one, it becomes evident that Peter is contrasting those who have been "born again" as believers versus those who are still spiritually dead. That those who are spiritually dead ought to get preached to as well so that they would have a fair chance to know the truth and live unto God, before they are judged.
Regarding the 1 Pet 3 passage, the 1 Pet 3 passages uses kerysso translated as "preached," which should actually be translated as proclaimed or heralded. The Messiah was basically giving the imprisoned angelic rebels a "status update" that he has achieved victory over the forces of darkness, having initiated the Kingdom on earth, and that their accomplices currently active are also doomed.
Personally, I believe it was to the souls of those who had already passed. The devils are appointed another day.