Carolyn Keene 
Whispers in the Fog [EPUB ebook] 

Katie Firestone, champion kayaker and owner of a whale-watching tour boat, has invited George, along with Nancy, Ned, and Bess, to vacation north of San Francisco in Seabreak. But when Katies boat is vandalized, suspicion falls on fisherman Holt Scotto, who may be trying to run Katie out of business. The menace doesnt stop there: a bike accident, a stalker, danger at an abandoned lighthouse, and a terrifying kayak trip lead Nancy to a hidden sea cave. Inside, Nancy discovers a dangerous secret. Soon her fearless instincts plunge her deeper into troubleand it will take all her courage, daring, and wits to get herself and her friends out of it alive!