The Tide Knot by Helen Dunmore

The second of the Ingo quadrology really picks up pace.

Sapphire and Conor have moved from their home, a tiny cottage by the sea to the little town of St Pirans to move in with their mother's boyfriend, diver Simon. Their mother wants to get all of them away from the memories in their former home, especially those of Saphy and Conor's missing father. But despite the noise of the surrounding human lives Conor and especially Sappy are drawn back to the sea and to Ingo, the world beneath the waves.

Sapphy increasingly finds herself with divided loyalties, pulled between the worlds of Ingo and Air and made increasingly aware of the sacrifices she and her loved ones would have to make if she decided to live permanently in one or the other. These come to a head in three incidents: Sapphy's hearbreakingly disappointing reunion with her father, a genuinely disturbing encounter; in the near death of Sapphy's loyal golden retriever Sadie when she tries to follow Sapphy into Ingo and her recovery under the hands of Granny Carne; and the unleashing of the the tides when the Tide Knot unravels. Completely compelling