Stormswept (The Ingo Chronicles) by Helen Dunmore

I wish I was away in Ingo, Far across the sea, Sailing over the deepest waters, Where love nor care can trouble me...Sapphire's father mysteriously vanishes into the waves off the Cornwall coast where her family has always lived. She misses him terribly, and she longs to hear his spellbinding tales about the Mer, who live in the underwater kingdom of Ingo. Perhaps that is why she imagines herself being pulled like a magnet toward the sea. But when her brother, Conor, starts disappearing for hours on end, Sapphy starts to believe she might not be the only one who hears the call of the ocean.