The Woman in Cabin 10 by Ruth Ware

Laura "Lo" Blacklock is excited when she's given the assignment of travelling on the debut cruise of the small luxury ship the Aurora. She figures it could be the big break she needs to move up at the travel magazine she works for. She arrives on the cruise with little sleep and frayed nerves after a burglar breaks into her flat a few days before her departure. Then, in the middle of the first night of the trip, she thinks that the young woman in the cabin next to hers has fallen overboard.  After she tries to report what happened, the blood she saw on the balcony has disappeared and she's told that no one was occupying that particular room. Is Lo correct in what she saw or could she be mistaken? The reader soon suspects that Lo belief is correct and she's probably in real danger, while her boyfriend at home is concerned that he has not heard from her in days...  The Woman in Cabin 10 is a locked room tale similar to The Girl on the Train.