The woman in cabin 10 / Ruth Ware.

"From New York Times bestselling author of the "twisty-mystery" (Vulture) novel In a Dark, Dark Wood, comes The Woman in Cabin 10, an equally suspenseful and haunting novel from Ruth Ware--this time, set at sea. In this tightly wound, enthralling story reminiscent of Agatha Christie's works, 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 and gray skies fall. Lo observes a woman in the cabin next to her, and one night hears what sounds like a body splashing into the water. Next day there is no record of the woman, Cabin 10 is locked up tight, and everyone thinks Lo is imagining things. 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.
A locked room mystery at sea