The basic problem with any post-apocalyptic novel is that it doesn't end. It just kind of picks a place to stop, and that's when we're done with our little group of survivors. The best scenes are the beginning, when the disaster, whether its, plague, zombies, or plague zombies, wipe out civilization, and then the rest of the book is a collection you've seen before: scavenging, the hope spot, the encounter with cannibal rapists, the last stand and so on. This one is no exception.
Where it does score points, and a lot of them, is the inventiveness of its apocalypse. The flood is an original concept, despite being a persistent problem in parts of the world and, y'know, being in the Bible and all. And instead of zombies, we get... well, it's tough to say, as the descriptions are intentionally vague. They come off like Lovecraftian blobs of protoplasm with The Thing-like ability to absorb people and then look like them. So that's at the very least interesting.
The survivors have a few bright spots in terms of characterization as well. From time to time the casts swells beyond the author's ability to handle it, but he quickly gets around to winnowing them down.
Final verdict: recommended for fans of survival horror, especially those with zombie fatigue.