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

through which waters the then world, deluged with water, perished.

through which the then world, by water having been deluged, was destroyed;

and that, by means of these, the then existing race of men was overwhelmed with water and perished.

And that the world which then was came to an end through the overflowing of the waters.

By water, the former world then, having been inundated with water, perished.

and that by the same means the world which then existed was destroyed in a deluge of water.

Whereby - Δι ̓ ὧν Di' hōn. Through which, or by means of which. The pronoun here is in the plural number, and there has been much difference of opinion as to what it refers. Some suppose that it refers to the heavens mentioned in the preceding verse, and to the fact that the windows of heaven were opened in the deluge (Doddridge), others that the Greek phrase is taken in the sense of (διὸ dio) "whence." Wetstein supposes that it refers to the "heavens and the earth." But the most obvious reference, though the plural number is used, and the word "water" in the antecedent is in the singular, is to "water." The fact seems to be that the apostle had the "waters" mentioned in Genesis prominently in his eye, and meant to describe the effect produced "by" those waters. He has also twice, in the same sentence, referred to "water" - "out of the water and in the water." It is evidently to these "waters" mentioned in Genesis, out of which the world was originally made, that he refers here. The world was formed from that fluid mass; by these waters which existed when the earth was made, and out of which it arose, it was destroyed. The antecedent to the word in the plural number is rather that which was in the mind of the writer, or that of which he was thinking, than the word which he had used.
The world that then was - Including all its inhabitants. Rosenmuller supposes that the reference here is to some universal catastrophe which occurred before the deluge in the time of Noah, and indeed before the earth was fitted up in its present form, as described by Moses in Genesis. 1. It is rendered more than probable, by the researches of geologists in modern times, that such changes have occurred; but there is no evidence that Pater was acquainted with them, and his purpose did not require that he should refer to them. All that his argument demanded was the fact that the world had been once destroyed, and that therefore there was no improbability in believing that it would be again. They who maintained that the prediction that the earth would be destroyed was improbable, affirmed that there were no signs of such an event; that the laws of nature were stable and uniform; and that as those laws had been so long and so uniformly unbroken, it was absurd to believe that such an event could occur. To meet this, all that was necessary was to show that, in a case where the same objections substantially might be urged, it had actually occurred that the world had been destroyed. There was, in itself considered, as much improbability in believing that the world could be destroyed by water as that it would be destroyed by fire, and consequently the objection had no real force. Notwithstanding the apparent stability of the laws of nature, the world had been once destroyed; and there is, therefore, no improbability that it may be again. On the objections which might have been plausibly urged against the flood, see the notes at Hebrews 11:7.

(5) Whereby the world that then was, being overflowed with (c) water, perished:
(5) Secondly he sets against them the universal flood, which was the destruction of the whole world.
(c) For the waters returning into their former place, this world, that is to say, this beauty of the earth which we see, and all living creatures which live upon the earth, perished.

Whereby the world that then was,.... The old world, as it is called in 2-Peter 2:5; and as the Ethiopic version here renders it; the world before the flood, that had stood from the creation 1656 years:
being overflowed with water; by the windows of heaven being opened, and the waters over the earth poured down upon it; and by the fountains of the great deep being broken up in it; thus by these waters from above and below, a general inundation was brought upon it; for that the deluge was universal is clear from hence, and from the account by Moses; for as the earth was filled with violence, and all flesh had corrupted its way, God threatened a general destruction, and which was brought by a flood, which overflowed the whole earth; for all the hills that were under the whole heaven were covered with it, and everything that had life in the dry land died, and every living substance was destroyed that was upon the face of the ground; see Genesis 6:11; and hence it follows, that hereby the then world
perished; not as to the substance of it, whatever alteration there might be in its form and position; but as to the inhabitants of it; for all creatures, men and cattle, and the creeping things, and fowls of the heaven, were destroyed, excepting Noah and his wife, and his three sons and their wives, and the creatures that were with him in the ark; see Genesis 7:23; and by this instance the apostle shows the falsehood of the above assertion, that all things continued as they were from the beginning of the creation; for the earth was covered with water first, and which, by the command of God, was removed, and, after a long series of time, was brought on it again, and by it drowned; and from whence it also appears, that this sort of reasoning used by those scoffers is very fallacious; for though the heavens and the earth may continue for a long time, as they did before the flood, in the same form and situation, it does not follow from thence that they always will, for the contrary is evident from what follows.

Whereby--Greek, "By which" (plural). By means of which heavens and earth (in respect to the WATERS which flowed together from both) the then world perished (that is, in respect to its occupants, men and animals, and its then existing order: not was annihilated); for in the flood "the fountains of the great deep were broken up" from the earth (1) below, and "the windows of heaven" (2) above "were opened." The earth was deluged by that water out of which it had originally risen.

Through which - Heaven and earth, the windows of heaven being opened, and the fountains of the great deep broken up. The world that then was - The whole antediluvian race. Being overflowed with water, perished - And the heavens and earth themselves, though they did not perish, yet underwent a great change. So little ground have these scoffers for saying that all things continue as they were from the creation.