Jilda's Ark by Verity Croker
What was supposed to be a sixteenth birthday celebration cruise for identical twins Jilda and Rosa quickly turns into a nightmare. While her mother and sister are enjoying an excursion on a Fijian island, Jilda, who is unwell and remains onboard, finds their ship has sailed off, leaving most of its passengers stranded on shore.
An unknown group has overtaken the ship. After a few days, they collect thousands of new passengers from a distant archipelago, making conditions onboard overcrowded and uncomfortable. Nobody seems to know where they are headed, so the mood on the ship rapidly deteriorates. Though Jilda is desperate to reunite with her family, she meets someone who comforts her and will completely change her world. But how will her new friend Jade feel when she learns Jilda has a boyfriend back home?