The Deep oleh Helen Dunmore

Sapphire lives in two worlds. On land she walks the rocky shores of the Cornwall coast—but under the sea she can swim like a seal by the side of her Mer friend Faro.

Now both of Sapphy's worlds are threatened. In the profound depths of the ocean, where the Mer cannot go, a monster called the Kraken is stirring. He has the power to sweep Ingo away and shake the land from its foundation.

Because of her mixed blood, Sapphire can enter the Deep. With a great whale as her guide, she will journey to a place so far from the sun, no light can find it—and confront an evil that's even darker.