I've read plenty of mermaid/men stories--there was another one nominated for a World Fantasy award this year, just as In Great Waters was. This book puts a very interesting new twist on it--the cross between "deepsman" and human is more fit for leadership than either purebred. That's the fantasy part, that human and merman can somehow interbreed.
The book builds somewhat slowly. Whitfield's characters, particularly the halfbreed royalty, are irascible and difficult to like. But build it does. All the types are fully described and realized, and by the end of the book you come to understand them. The royals seem more human than the humans, in the end.
It doesn't seem fated to last--as the clever humans develop technology, they seem likely to push the deepsmen aside entirely. But it's entertaining to think about.