An astounding book, highly recommended. Takes what seems light the most silly, bending-suspension-of-disbelief premise--mermaids as the central factor in the transformation of European society--and weaves an intense and satisfying story from it.
Whitfield is perhaps one of the ultimate subverters of the Masquerade, and whole theme of hidden fantasy within the present world. Her scenario takes just the opposite assumption--posit the existence of a fantasy element, even one as detached and minor as mermaids and it eventually transforms everything. In her novel contact with "deepsmen" and humans around the Renaissance lead to one Venitian family brining them openly into full on political and military conflict, leading to widespread rise and fall of different dynasties across Europe.
It is then a fantasy novel without magic, but with the presence of non-humans leading to a full-fledged alternate history that alters a lot of patterns for intrigue, culture and daily life. In Great Waters isn't about how that change is established. This element is given as backstory, instead it focuses on a specific regional situation several hundred years later where the change has already been made and everyone accepts the uses of human royalty linking politically to deepsmen and the level of intrigue for both species is deeply intertwined. It's refershing to see a strange fantasy situation taken for granted, as part of the enviornment that people have to work within. Characters in the novel find the arrangement inconvenient at points, but it's an accepted part of the sider environment with a lot of practical force behind it. The danger isn't that spurring an alliance with deepsmen will cause, for instance, England to get wiped out by them--even for an island country the water environment doesn't pose that kind of existential threat, there's simply no incentive for the deepsmen en masse to go up on land. Rather, the danger is that France with an alliance of humans and deepsmen will gain a compelling advance in navy and commerce, and that this development will lead to the decline of England's political position. In a way the book works precisely in deflating expectations of wonder and awe, by having characters accept their world largely as it is, as the unpredictable but rational situation they are accustomed to. By refraining from the moments of horror or amazement at the setup of the co-existing environments (although there is a lot of surprise when water and land societies co-mingle directly) it brings a greater sense of credibility to the setting, while also emphasizing awe in the reader for the lengths of worldbuilding.
In another strength the deepsmen are entwined with humanity but they haven't become human, and in the story told half from the point of view of one we see a very different frame of reference. It's not intrinsically better than humanity or worse--it's a psychology framed by a different environment that has a different take on basic things like family, sex and competition. One way this contrast comes up is religion--the deepsmen character, when required for social position to profess at least a nominal acceptance of God and the church refuses flat out, considering the whole concept absurd. Whitfield doesn't do the obvious with that setup, and refrains from either endorsing a theist or atheist viewpoint. Instead that framing is used to explore how separate the two viewpoints are in this case, how different the non-belief is from the human types of rejection of religion that generally occur. Highly different and yet symbiotic, such is the situation between the two species and it makes for a very stron gpremise.
The tone of the story is unsentimental and at points rather cold, and it refuses to implement the type of romantic atmosphere one would expect from the drawn out relationship at the core of the book's structure. It's not a depressing or particularly pessimistic book though, and feels more like a realistic exploration of the strange point of departure, with a range of accompanying human virtues, vices, betrayals and loyalties. Excellent writing, very engaging narrative, and some very creative ideas at work. However the point I found it most effective was in characterization, making an overall atmosphere, and showing a protagonist that doesn't share common human assumptions in a number of ways. There's a pleasing ambiguity across most of the book, it being quite unclear what will happen next or how the larger issues of the setting will play themselves out.
I'd say this book is up there with the very best of 2009, and it made it onto my Hugo nomination list for the year. Didn't get on, though, more's the pity. This point was one in my reading where I was reading a lot of recommendations for unfamiliar authors in fantasy and science fiction, and as the last half-dozen posts shows a number of these had modest or significant failure. It was good to hit a stark, flat out excellent book, one that shows speculative fiction can blow the lid off our assumptions and normal patterns and deliver a story extremely unique and strange, but at the same time fully coherent and 'realistic'.
Better than: Legacy by Greg Bear
Worse than: The Scar by China Mieville.