In Great Waters — Kit Whitfield

During a time of great upheaval, the citizens of Venice make a pact that will change the world. The landsmen of the city broker a treaty with a water-dwelling tribe of deepsmen, cementing the alliance through marriage. The mingling of the two races produces a fresh, peerless strain of royal blood. To protect their shores, other nations make their own partnerships with this new breed-and then, jealous of their power, ban any further unions between the two peoples. Dalliance with a deepswoman becomes punishable by death. Any "bastard" child must be destroyed.
This is an Earth where the legends of the deep are true-where the people of the ocean are as real and as dangerous as the people of the land. This is the world of intrigue and betrayal that Kit Whitfield brings to life in an unforgettable alternate history: the tale of Anne, the youngest princess of a faltering England, struggling to survive in a troubled court, and Henry, a bastard abandoned on the shore to face his bewildering destiny, finding himself a pawn in a game he does not understand.
Yet even a pawn may checkmate a king.