Helen Dunmore 
Ingo [EPUB ebook] 

There is a legend in Cornwall of a man who fell in love with a mermaid, a man who swam down into the sea one night and met his Mer love. He was never seen again. Sapphire knows the legend well. Her father disappeared at sea, and now her brother, Conor, keeps vanishing by the shore, too. Sapphy also feels the inexorable lure of the ocean, a temptation that reveals the truth of the legend and opens a beautiful world beneath the waves: the enchanted, undersea realm known as Ingo. But there’s a dark and dangerous side to Ingo, and Sapphy must face its power or lose touch with everything-and everyone-she loves on land.