Sleepers. The Blue Planets World Series, #1

Ages 9-14Could there be alien sleeper cells on Earth?He comes from an alien world. But he's the best hope for Earth. In this coming of age science fiction story, Jake Rose-who is half Earthling and half alien-must discover the truth of what the Risonian aliens have been doing on earth. He joins his human grandparents on Bainbridge Island, but finds alien sleeper cells hiding in Puget Sound. They're desperate! Their world is dying, and they need a new planet.
They can breathe in air or water, so they only want to live in Earth's oceans. Jake must stop those-on both sides--who would sabotage the Risonian's only hope for survival. But strangely, he finds that he's also Earth's only hope for survival. ".