Peter reminds us that the earth was composed of water and through water. According to the Genesis story, in the beginning, there was a kind of watery chaos.
So out of this watery chaos, the world was formed. Also it is through water that the world is sustained because life is sustained by the rain that comes down from the skies. So Peter is pointing out that the world was created out of water and is sustained by water, and it was by the same element that the ancient world was destroyed.
The warning that Peter is giving to the church is that these scoffers reason like this: As things are so they will be forever, nothing changes. That is what we say about our politicians and their methods of rule in one sense. But it will all change, thankfully. They build their hopes on the idea that this is an unchanging universe, but of course, they are wrong because the ancient world was formed out of water and is sustained by water and it perished in a watery flood.
