The Devil and the Dark Water

This novel by Stuart Turton is an historic novel, I guess, or as The Guardian describes it, "A maritime mystery with fantastical overtones." The two protagonists are "Arent Hayes, a physically imposing specimen with a kind soul and a 'poisoned' past, and healer Sara Wessel, abused wife of soulless Governor General Jan Haan, who happens to be Arent's uncle" (Kirkus).  I am copying the Publishers Weekly review since it does a better job of summarizing the plot than I could. "Set in 1634, this outstanding whodunit from Turton (The 7½ Deaths of Evelyn Hardcastle) opens in Batavia, where celebrated investigator Samuel Pipps, who was working in the Dutch East Indies until his arrest for an unknown reason, is about to be transported to Amsterdam aboard the Saardam, along with his longtime sidekick, Arent Hayes. From the dock, a bloody man issues a dire warning to the Saardam's crew and passengers. As the grim figure, who appears to have leprosy, prophesies that the ship won't reach its destination, his clothing bursts into flame. Hayes and another passenger, the governor-general's wife, rush to help the dying man, only to find that his tongue had been cut out, making any speech impossible. The puzzles only continue once the vessel sets sail, including a locked-room murder, the reappearance of the dead leper, and a ghost ship dogging the Saardam. As Turton ratchets up the tension en route to the brilliant resolution of the plot, he keeps readers in doubt as to whether a rational explanation is possible. Fans of impossible crime fiction won't want to miss this one." 

Kirkus also comments that Turton "brings a pointed social conscience to bear in his commentary on the ill treatment of women and the exploitation of the lower class," and concludes that he delivers "A devilish sea saga that never runs out of cutthroat conspiracies." The Guardian says of the author's skill, "Turton has got his world up and running inside the first two pages; thereafter, deceptions and diversions multiply until the ultimate, outrageous reveal, at which point the dark water turns out to be rather darker than you imagined."