Water like a stone / Deborah Crombie.

Enhanced descriptions from Syndetics:

When Scotland Yard superintendent Duncan Kincaid takes Gemma, Kit, and Toby for a holiday visit to his family in Cheshire, Gemma is soon entranced with Nantwich's pretty buildings and the historic winding canal, and young Kit is instantly smitten with his cousin Lally.

But their visit is marred by family tensions exacerbated by the unraveling of Duncan's sister Juliet's marriage. And tensions are brought to the breaking point on Christmas Eve with Juliet's discovery of a mummified infant's body interred in the wall of an old dairy barn--a tragedy hauntingly echoed by the recent drowning of Peter Llewellyn, a schoolmate of Lally's.

Meanwhile, on her narrowboat, former social worker Annie Lebow is living a life of self-imposed isolation and preparing for a lonely Christmas, made more troubling by her meeting earlier in the day with the Wains, a traditional boating family whose case precipitated Annie's leaving her job.

As the police make their inquiries into the infant's death, Kincaid discovers that life in the lovely market town of his childhood is far from idyllic and that the dreaming reaches of the Shropshire Union Canal hold dark and deadly secrets . . . secrets that may threaten everything and everyone he holds most dear.