Apostle Peter   spoke in 2 Peter 2: 5 about the flood in the context of  false prophecies and false prophets. His discourse began in 2 Peter 1 in which he exhorted the believers to hold fast to the truth after his imminent  death. He reminded them that they were not taught “cunningly devised fables” (Greek mythology etc?), but had received salvation from those who were eyewitnesses of the truth.    

Peter linked the swift destructions of the flood and Sodom and Gomorrah with  false prophets  whose “damnable heresies” and “pernicious ways” denied the Lord (2 Pet 1: 20-  21 through 2: 1-7):

Knowing this first, that no prophecy of the scripture is of any private interpretation.

For the prophecy came not in old time by the will of man: but holy men of God spake [as they were] moved by the Holy Ghost.

But there were false prophets also among the people, even as there shall be false teachers among you, who privily shall bring in damnable heresies, even denying the Lord that bought them, and bring upon themselves swift destruction.

 AND  many shall follow their pernicious ways; by reason of whom the way of truth shall be evil spoken of.

AND  through covetousness shall they with feigned words make merchandise of you: whose judgment now of a long time lingereth not, and their damnation slumbereth not.

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;

AND  turning the cities of Sodom and Gomorrha into ashes condemned [them] with an overthrow, making [them] an ensample unto those that after should live ungodly;

And delivered just Lot, vexed with the filthy conversation of the wicked

An initial, cursory review of 2 Pet 2 would lead us to believe that the verses are isolated and unrelated to each other.  However, Apostle Peter  also used the conjunction “AND” at the beginning of  most of these verses. Dictionary.com says that a conjunction is “used to connect  words, phrases, or clauses”.   The phrase  will therefore  eventaully take on the meanings  “along or together with; as well as; in addition to…” 

The Apostle used the  preposition “for”  at the beginning of sentences to show that the verses are linked or are connected to each other. Dicitonary.com also says that “for” can also be used as a  conjuction to carry the meaning ”  seeing that, since, because”.

We can therefore confirm that fallen angels were responsible for the heresies that led to the destruction of the old world by flood as well as Sodom and Gomorrah.  Apostle Peter’s letter is current for any era and we need to heed the truth and escape the damnable heresies that Apostle Paul called the “doctrines of devils” (1 Tim 4: 1). Satan has not ceased his quest to lead men astray from God. Let us therefore hold fast to the Eternal truth of God’s word.