Deepsmen and Landsmen can 'interbreed' (you will forgive me using a term like this, but in this universe it makes sense, as it does not when talking about so-called race in our universe). In the 10th century a family of hybrids, who are both intelligent and robust, took over rulership of Venice, and now all the royal families of Europe are descended from this mixed lineage, controlling both land and sea. The royal family ruthlessly destroys any other hybrid children who are born, as potential usurpers.
The story begins with a hybrid child living in the ocean and finding it hard to survive. The deepsmen have a very simple animal-like social order, with little place for a child who can't swim fast. His mother abandons him on the beach, and he is taken in by an aristocratic family who plan to use him as a pretender to the English crown.
The overall tone is very realistic and grounded. It niggled a wee bit with me that if she'd just tweaked the biology a little, it would have been much more scientifically plausible, but I don't mind suspending disbelief. The feeling of Mediaeval politics and religion is very authentic, and the contrasting feeling of the two worlds, land and sea, is beautifully conveyed. The story reminded me of Wolf Hall. It would be hard to think of a better example of an historical fantasy AU, so if you like that kind of thing, this is a very good example.
Most interesting to me was the personality of the mixed-species hybrid people. They are a little like the ancient-breed hero of 'Black Man', or the old SF tradition of Slans. They stand a little outside of the social ease of either sub-species of humans, and they find religion and morality problematic in various ways. They are vividly drawn, plausible, and likeable. In fact the more I think about it, this is definitely the long standing SF trope coming out, to appeal to the common feeling of both competence and outsiderness, and very well done in its type.