Land's End — Frederik Pohl

When Comet Sicara brushed near enough to strip the ozone layer form the Earth's atmosphere, civilization effectively ended--in fact, life on Earth was nearly extinguished. But the underwater cities survived, and some heavily protected land enclaves as well. When the "ozone summer" years were ending, submarine captain Ron Tregarth rediscovered his lost love, Graciela Navarro. but their triumph against all odds was only the beginning, for the alien known as the Eternal stood between them and threatened to destroy all they held dearest. The Eternal's goal was to absorb the minds of every living thing, to create a death-in-life to enslave the planet.