Devil and the Deep Blue Sea by T.S. McKinney

Living on the shores of the surface world for the past seven years, Kailar has no idea who he really is—not only a Fae creature but a Mer Prince, who was taken from his home in the sea for his own safety and forced to live in his human form. All his memories of his former life were wiped away by the one person who should have protected him—his soulmate and an Atlantean warrior, Lord Alyxsander. When he becomes desperately ill from being away from the ocean too long, Alyxsander finds him and tries to make up for the time they’ve lost. Kailar is furious, however, and refuses to forgive him, wanting only to return to his home to find his long-lost twin brother, Adan.
He learns that Adan is in terrible danger, being held captive by their malicious mother and Kailar just may be the only one who can save him. But first, Kailar must learn the full potential of his true destiny and somehow find his way back to his family and his one true love in order to save his life and the future of the Mer people.
Unforgettable characters, a battle with sea monster, a sea hag, and a memorable love story…will Kailar survive in this underwater realm long enough to find his happily ever after? The Devil and The Deep Blue Sea explores a fantasy world that delves into the magical world under the Sea.