The Turning Tide — Catriona McPherson

It is the breezy Scottish summer of 1936, Lady Dandy Gilver has been called, with trusted colleague Alec Osbourne, to solve the strange case of the Crammond Ferrywoman on the Firth of Forth.
A small island is home to a woman, Vesper Kemp, who has lost her mind, spending her days rambling in rags.
What is more troubling, is that Vesper claims to have murdered a young man. A concerned group of residents have good reason to believe she is innocent. But Dandy and Alec will have a dangerous journey ahead if they are to uncover the truth in the River Almond's murky waters.