20 The antecedent is ambiguous. It could refer to the heavens, the heavens and earth, or the water and the word. If the reference is to the heavens, the author is reflecting on the Genesis account about “the floodgates of the heavens” being opened (Gen 7:11). If the reference is to the heavens and earth, he is also thinking about the cosmic upheaval that helped to produce the flood (Gen 6:11). If the reference is to the water and the word, he is indicating both the means (water) and the cause (word of God). This last interpretation is the most likely since the final nouns of v. 5 are “water” and “word of God,” making them the nearest antecedents.

In other versions

and that by means of these the world that then existed was deluged with water and perished.

By these waters also the world of that time was deluged and destroyed.

Through these the world of that time perished when it was flooded.

Then he used the water to destroy the ancient world with a mighty flood.

by means of which things the world that existed at that time was destroyed by being inundated with water.

Whereby the world that then was, being overflowed with water, perished:

Public Domain