The Woman in Cabin 10 book by Ruth Ware

Lo Blacklock, a journalist who writes for a travel magazine, has just been given the assignment of a lifetime: a week on a luxury cruise with only a handful of cabins. The sky is clear, the waters calm, and the veneered, select guests jovial as the exclusive cruise ship, the Aurora, begins her voyage in the picturesque North Sea. At first, Lo's stay is nothing but pleasant: the cabins are plush, the dinner parties are sparkling, and the guests are elegant. But as the week wears on, frigid winds whip the deck, gray skies fall, and Lo witnesses what she can only describe as a dark and terrifying nightmare: a woman being thrown overboard. The problem? All passengers remain accounted for and so, the ship sails on as if nothing has happened, despite Lo's desperate attempts to convey that something (or someone) has gone terribly, terribly wrong.