A third view, apparently the oldest, identifies these "spirits in prison" with fallen angels, equated with "the sons of God" in Genesis 6. This view was widely known and generally taken for granted in the apostolic age. It is strongly presented in the Book of Enoch, a composite pre-Christian, Jewish apocryphal writing widely known in the early Christian church., This view fell into disfavor with the fourth-century church.12 This angelic trans-gression was always viewed as having taken place just prior to the Flood. Proponents point to 2 Peter 2:4-5 and Jude 6 as evidence that this view was known and accepted in the early Christian church. They also point out that in the Gospels the word "spirits" frequently refers to supernatural beings (Mark 1:23, 26, 27; 3:11; 5:2, 8; etc.). The only clear instance in the New Testament where "spirits" is used of the surviving part of man after death is in Hebrews 12:23, but this is immediately indicated by the addi-tion "of righteous men made perfect."13 References to "spirits" as supernatural beings, either good or bad, occur in the intertes-tamental literature, for example, Tobit 6:6; 2 Maccabees 3:24; Book of Jubilees 15:31; The Testament of Dan (in The Testa-ments of the Twelve Patriarchs) 1:7; 5:5. This view seems to be supported by the teaching in 2 Peter 2:4 and Jude 6  and is therefore the most probable.
See http://faculty.gordon.edu/hu/bi/ted_hildebrandt/NTeSources/NTArticles/BSac-NT/Hiebert-1Peter3-Pt2-BS.htm for the entire exegesis of 1 Peter 3:18-22.