The House at Elder Carr. Tide House Mysteries, #3

For thirty-four years, an old woman on the marsh wrote letters to a sister no one else remembered. She never sent one - she only had to keep them, against the day someone would come. When Edith Vexley dies, her daughter finds the record of a lifetime, and the reason her cousin Peter drowned eight weeks before her, in water he was terrified of, in a death everyone agreed was an accident. Now a Detective Chief Inspector, Honor Whitling knows an accident near water when she sees one.
The third Tide House Mystery.