This confirms who these ‘spirits in prison' were. Human beings are never spoken of in this way (as spirits in prison), while 2 Peter 2:4 confirms Peter's interest in the angels who sinned in the time of Noah. And we should note that their disobedience and its punishment had taken place against the background of another time when the longsuffering of God was waiting for a response from a sinful people who were under the sway of demonic powers, and when there were eight righteous people who alone were obedient and preached righteousness (2 Peter 2:5). In accordance with God's instructions they built an ark, and all the time that they were building it God in His longsuffering was giving an opportunity for men to repent. For God is longsuffering. He is not willing that any should perish but that all should come to repentance (2 Peter 3:9). But although He was patient none but the eight responded. However, whatever their sufferings they came through it, and in that ark they were saved ‘through water', which as it were lifted them up to God, while at the same time that same water drowned the remainder, and the spirits were put in prison.
Related Commentaries of 1 Peter 3:19
1 Peter 3:19-20
19 By which also he went and preached unto the spirits in prison;
20 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.