Deep Wizardry — Diane Duane

What's supposed to be just a plain old summer vacation -- swim, sand, and sun -- takes a turn for the magical when Nita and Kit come to the aid of a fellow wizard. Only, this wizard is a whale, and she needs the two teens to join a group of whales and dolphins in an ancient underwater ritual. But performing the Song of the Twelve is not easy -- and there are things in the ocean more dangerous than even the Lone Power, such as the enormous Master Shark. He is as old as the sea and has his own role in the Song of the Twelve, a role that requires only that he do what he's best at...eat someone.