I've read the first book, and I'm reading the second one, The Tide Knot, which was only published in May. I was thrilled with the story.
Ingo
Living on the Cornish coast with her mother, father and brother Conor everything is great for Sapphire. That is, until, Sapphire's father goes out in his boat in the cove, and goes missing. And then, who's the strange girl on the rocks, talking to Conor on hours on end?
After finding another boy who looks like Conor, who's not him, on the rocks one day, Sapphire discovers the world of Ingo. Ingo's a haunting, dangerous world under the waves. They go riding all sorts of currents, wheather strong or weak, learn to talk to seals and riding on the back of dolphins, unaware of the harm that divers can do.
One day, a man that Sapphire's mum likes and that her father would take as a rival, appears and teaches Conor how to dive. His name is Roger, and Sapphire strictly hates him.
On Roger's first night, Sapphire can hear a strange voice like the waves at the back of the house, calling her name....