Flood — Stephen Baxter

Now the bestselling author puts both of those attributes to work in a novel as frightening as it is plausible....

Four hostages are rescued from a group of religious extremists in Barcelona. After five years of being held captive together, they make a vow to always watch out for one another. But they never expected this....
The world they have returned to has been transformed -- by water. And the water is rising.

As the water continues to flow from the earth's mantle, entire countries disappear. High ground becomes a precious commodity. And finally the dreadful truth is known: Before fifty years have passed, there will be nowhere left to run....