So the only possible explanation of the cause of the Waterworld apocalypse is earth being hit by millions of giant ice meteors.
But on the other hand, society had collapsed along with most of the stored knowledge of the world, so maybe it was just the people being uneducated and highly insular. They don't even know how long a kilometer is anymore.
There was a point in history a few hundred million years ago where the entire American midwest was a shallow water ocean; we could easily be heading back to that scenario in just a few centuries.
The only hard sci-fi explanation is that someone towed Europa to Earth and slowly lowered it down via a cable or pipe. Terraforming by an alien aquatic species. Hyperintelligent space whales.