ICEBREAKER (The Icebreaker Trilogy, 1) by Lian Tanner

Twelve-year-old Petrel is an outcast, living on an ancient icebreaker that has been following the same ocean course for three hundred years. The ship's crew has forgotten its original purpose and has broken into three warring tribes. Everyone has a tribe except Petrel, whose parents were thrown overboard for alleged crimes. She has survived by living in the dark corners of the ship, and speaking to no one except two large rats, Mister Smoke and Mrs. Slink. When a boy is discovered on a frozen iceberg, the crew is immediately on alert. Petrel hides him on board, hoping he'll be her friend. What she doesn't know is that the ship guards a secret, held down deep in its belly, and the boy has been sent to seek and destroy it. Icebreaker by Lian Tanner is a lush fantasy and thrilling adventure story, with an unforgettable friendship at its heart.