Deep Wizardry - Diane Duane
Coming to the aid of a wounded whale, Kit and Nita are plunged into deep wizardry. The whale is a wizard, and she enlists Kit and Nita in battle against the sinister Lone Power. Becoming whales themselves, Nita and Kit join in an ancient ritual performed by whales, dolphins, and a single fearsome shark. But which poses more of a danger: the Lone Power, or ed'Rashtekaresket, the enormous shark as old as the sea?