The story of a great flood drowning the world is an interesting premise, and could have been a truly fantastic read. Baxter makes a dutiful effort in walking us through the shock felt by the near-Earth's inhabitants when the waters rise and don't subside. I enjoyed the set-up for his protagonists, as they, too, have been separated from this world by a few years and can therefore justifiable be confused. As they learn, we learn, and it helps to avoid exposition dumps (at least in the beginning of the novel).
as we progress, however, it becomes apparent that there isn't really a point to the story, except perhaps 'humans deserve it'. We don't see the human cost of suffering - we're always with the survivors, and they don't seem to spend much time thinking about anyone they've lost. Main characters are killed out of sight, and the constant influx of dozens of new characters, all given equal weight, is disorienting. The human relationships become more and more unbelievable as the story progresses, with mothers refusing to talk to their children even in this drowned world because of who they shack up with, people being passed around like objects, and allegiances changing every chapter. Most frustratingly, a lot of weight is placed mid-way through until the end on the relationship between our protagonist and one of her former hostage friends. A romantic relationship is manufactured out of thin air, and we are later informed that the middle-aged man is in fact in love with the protagonist's niece. Given that the last time we met said niece she was 16, that's a little creepy. (This also follows some other suvivalist also trying to walk off with the girl, a la '28 Days Later'). His 'love' continues through him killing people she loves, and everyone around him tolerating this kind of behaviour because 'he can't help it'.
Meanwhile, billions of people die in between chapters, unmentioned and unnoticed.
Baxter's idea is strong, solid. There was real potential there, especially in the first third. But his relationships are unconvincing, and there is too much pseudo-science here for the book to hold water (!) otherwise. Most of all, he tries to turn a human tragedy into a political discussion by focusing on the most privileged strata, those that somehow always come out on top. What would have been much more interesting would be more of a focus on the less-fortunate, those that have to struggle to survive, rather than simply be affiliated with a multinational.