The Message (Animorphs #4)

It all started with the dreams. But Cassie didn't pay much attention to them. She and her friends have been having nightmares ever since they acquired the power to morph. But when Cassie discovers that Tobias has been having dreams, too — the exact same dreams — about the ocean, and a voice that's calling to them for help, she decides it's time to start listening. Now she and the others have to figure out if the dreams are a message, or a trap.