Helen Dunmore 
Deep [EPUB ebook] 

A devastating flood has ripped through both worlds, Air and Ingo. And now, deep in the ocean, a monster is stirring and demanding his due. Mer legend dictates that only those with mixed-blood-half Mer and half human-can conquer the Kraken. So Sapphy must return to the deep and, with the help of her brother, Conor, face a struggle that will test her like no other.