Not a great book. A decent book, yes, but not a great one.
During a great period of space colonization, the planet Earth sent residents of a region of Russia to populate a planet entirely covered by water. They would have given the planet a miss, but it's rich in minerals they need back on good old mother-Earth.
The aquatic geography of the planet is a key element in the story, in that everything has a naval feel to it. The people live in big underwater cities, and they travel everywhere by submarine. The claustrophobia and aquaphobia nodes in my brain tingle just thinking about it. But the maritime setting of the book actually leads to one of my complaints: there is so much navy-speak and so many technical terms and equipment related to water travel that I wasn't familiar with. The character Kane spends a good deal of time explaining Earth terms for the benefit of Katya, but the author doesn't do nearly as good a job explaining naval terms to the reader. As another reviewer pointed out, some of these terms and ideas (including the technology aboard the Leviathan) are so crucial to the action in the book that the reader needs to understand them well. I didn't understand them, so I found myself confused at points where the action was at its busiest.
I also was disappointed in some of the character development (or lack thereof). The author vaguely mentions that Katya's father died in the Terran-Russalkan War, and that her mother died as a result of an accident, and that's why she lives with her Uncle Lukyakin, but he doesn't give us much detail. Lukyakin himself fought in the War, and that's apparent when his battle skills resurface later in the book, but we don't know much more about his past. Kane has all kinds of issues and secrets, and some of those secrets are revealed, but things brought up early on are just left to fade away. (At one point, when it looks like Lukyakin has died, Kane offers sympathy to Katya. Katya tells him there's no way he could know the pain she's experiencing; his face blanches and he practically runs from the room. When another character learns what Katya has said, she implies that Kane has undergone huge tragedies himself, but we never hear more about it.)
Because the characterization is so simplistic, but the technical aspects are so advanced, there seem to be two books - one is a basic young adult dystopian novel, while the other is a more advanced science fiction novel. Neither of them is particularly well-written. And ultimately, that may be why - though I like the book - I don't *love* the book.