Kraken Wakes is much like The Day of the Triffids in style and development. It's an apocalypse that develops slowly at first, and then suddenly there's a tipping point and civilization as we know it could end. Mysterious fireballs from space land in the ocean. There is soon evidence that an intelligent deep-water species has invaded Earth. Ocean-going ships occasionally disappear, and human retaliate with blind deep-water bombing. The escalating conflict leads to raids on coastal areas by the deep-water, blob aliens, and then, after the raids are successfully repelled, the aliens melt the polar ice caps and flood the world.
One of Wyndham’s main themes is that science, which has rendered nature largely familiar and predictable, has also made us incapable of contemplating catastrophic change in spite of the evidence that catastrophic change has occurred in history. (I'll resist explicitly discussing the parallel here with the catastrophic change threatening us today.) The main scientist character in the book, Dr. Bocker, is a Cassandra who argues early that the world is facing a major threat, but he’s naturally ridiculed as an alarmist.
As the crisis worsens by steps (first ocean travel is rendered unsafe, then coastal raids are a danger, and finally, the sea level slowly rises), society refuses to recognize the threat, or when it does, is confident that a scientific solution is just around the corner. People are unwilling to contemplate that the terms of humanity’s existence on Earth are about to change.
Thus, Kraken Wakes solidly takes on one of the big themes of the End of the World genre. As in Triffids, the story is told with controlled, artfully constructed prose, which makes Wyndham one of the most enjoyable writers of 1950’s (post)-apocalyptic sci-fi.