The Castle of the Demon — Patrick Ruell

A woman on the run from her husband encounters danger in a seaside town: “An eminently likable heroine . . . a highly clever ending” (Booklist).
 
Emily Salter is once again using her maiden name, living with her dog in a town along northern England's coast that she remembers fondly from childhood holidays. Her intention was to get far away from her controlling and threatening husband. Here, she's found some refuge, even met a nice gentleman, though she's not anxious to jump into another relationship.
 
But her new home is not as peaceful as it appears on the surface. Spies lurk in its remote corners -- and a dead body lurks under the water. To Emily's shock, she is beginning to suspect that the intrigue has something to do with her -- as well as the man she thought she left behind -- in this suspenseful mystery by a Diamond Dagger�"winning author who “delivers literate, complex, and immensely satisfying thrillers”( Orlando Sentinel).
 
Also published under the title The Turning of the Tide.