Deep Waters by Thom Collins

In search of a story, he found murder and romance.
Author Christian Costner is researching material for one of his dark thrillers, and Nyemouth seems like the perfect setting for his next book. The small seaside town has witnessed plenty of trouble over the years, and Christian thinks it will provide him with the inspiration he needs.
He hires local tour guide and fisherman Harry Renner to help him explore the coastline for a couple of days. Harry is knowledgeable and mature beyond his twenty-eight years. Handsome, too, though Christian thinks Harry is far too young for him.
As the weather worsens, Harry cuts short their first sightseeing trip. Heading back to shore, they spot a figure in distress in the water. A difficult rescue is made far worse when they discover that the casualty has a knife wound to his abdomen and dies before they reach the safety of the harbour.
United by the trauma, Christian and Harry find comfort in each other, but when another murder comes to light, they find themselves at the heart of a dangerous mystery and the target of a killer more ruthless than they could ever imagine.