Note* If, in in the reader's mind, "sons of God", as used in Genesis 6, means "angels", you either won't understand or won't accept any of the following:
The "spirits" were "in prison" when Jesus "went" and preached to them. The Bible says that God "chose the foolishness of preaching to save them that would believe". It also says "by which" meaning "by being put to death and quickened by the spirit", he "went"(it was made possible by death) and "preached" to "spirits" which were already "in prison", not living beings as would have been if the preaching was done before the flood. And I see no need to "preach" to "angels" which have already "sinned" and are already reserved for judgment.
The "spirits in prison" could be the "sons of God" of Genesis 4,5 and 6. We know than men "began to call upon the name of the Lord" back in Genesis 4 (Genesis 4:26) and so we know there were believers. Furthermore, we see that there were believers, named in Genesis chapter 5, who are direct ancestors in the lineage of Christ(Luke 3:36-38). We also can determine that at least two of these "sons of God", Lamech and Methuselah, would have been alive while "the ark was a preparing". One was Noah's Grandfather and the other was his own father; So why does it not mention their helping Noah in his endeavors?
Then there is the matter of "becoming mighty men". In Geneis 6:4. The verse does not specify which of the groups, the "sons of God" or the "children" became "mighty men": It only says "the same became mighty men" . But think about it: if you marry more than one wife, or even just one, "beget sons and daughters" for hundreds of years, you would still be around to witness your family multiply astronomically during your lifetime. Your family would be a mighty nation: a "superpower" if you will.
Genesis 6:4 says "There were giants in those days" followed by a semicolon, then it says and "also after that". The term "also after that", in this context, cannot mean "later in time" because the whole story takes place "in those days". In this context, the word "after" , as used in "and also after that", can only mean essentially the same as it does in "after his kind". It means the "sons of God" and their families were becoming "mighty men" "after" (meaning "in conformity with" or "as a reaction to") the presence of giants. They conformed to a world "filled with violence" (Genesis 6:11). That must have been part of their sin of "disobedience".
In conclusion to the above, I believe that this preaching was done in the spirit, during the three days after Jesus died on the cross. I don't believe he would have preached to any angels or any of the wicked dead. He preached to believers who had been "disobedient" before the flood.
