Isabel of the Whales

Eleven-year-old Isabel is a "plain old" girl living in Provincetown, Massachusetts, who believes that she is destined to accomplish something special. When her fifth-grade class goes on a whale-watch field trip, something amazing happens: Dozens of different species of whales surround the boat, bumping the deck and sending Isabel flying into the ocean. Isabel is shocked to hear the whales speaking to her--she is a mermaid, they tell her, a "Chosen One" who has the ability to turn from a human into a whale and back again. She is destined to live among the whales long enough to learn their ways, and teach them about the human world.
Living among her pod is fun, at first, but Isabel has an important mission. She will change the whales' future forever, and learn a lot about herself in the process.

From the Hardcover edition.