The flood served two purposes because it was also a judgment for those living on the planet. Once the fallen angels dissolved their fleshly containers and were back in the spirit realm, they were stripped of the power to materialize again.
As mankind grew in numbers, Satan tempted some of the angels away from God's organization. They materialized on this planet and married human women. They told the people of the preflood days what secrets they knew of heaven. The marriages between the angels and humans did not please God. Noah and his family were righteous in God’s eyes, so they found favor with him. They were the only humans to survive the flood. The waters of that flood not only drowned all living things that resided on previous dry ground, but it also took with them the secrets divulged by those fallen angels.
The flesh the angels materialized was not what God had started the first human pair from. It was a counterfeit flesh that doomed the offspring to a judgment of destruction, as well as the wives of both the fallen angels and the Nephilim offspring. The angels that materialized in human form were not of mankind’s "one flesh". They materialized a counterfeit flesh that did not represent God's original creation, and the union between these counterfeit humans and the female offspring of Adam and Eve produced their own hybrid offspring.
The fallen angels kept some of their superior attributes while taking fleshly form, and this produced the Nephilim sons that became "mighty ones." Something else that made them stronger and mightier was that they ate animal flesh, which was not on mankind’s menu back then.
It was God's will that none of them should survive the deluge, but, he also wanted the disobedient angels to return to the spirit realm. The flood accomplished that. For their transgression, the fallen angels were put under Satan’s control, because he was the first of his kind to sin.