Deep Wizardry - Diane Duane

During a summer vacation at the beach, 13-year-old wizard Nita and her friend Kit assist the whale-wizard S'reee in combating an evil power. 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?