Sandpiper Marsh by Vickie McKeehan

His sister’s disappearance haunts Ex-Navy SEAL Beckett Callahan. For years, all roads have led to nothing but dead ends. To cope, he and his brother Birch—another ex-Navy SEAL—run Terra Search & Recovery to help other families bring closure to their pain. He even hopes to train his golden retriever Brodie to become a search dog with the Coastal Canine Unit.
After multiple tours overseas, Beckett has finally managed to rebuild his life. But failing to find out what happened to his sister overshadows everything. That changes when Beckett meets Dr. Kelly Ecklund, a quirky marine biologist, in town to reforest the dying kelp beds. As sparks fly and attraction builds, is it enough to bring him out of his dark shell? Will Kelly’s interest in the case open up old wounds or be the catalyst that heals an entire family?