Sins of Silence by Valerie Wilcox

Sailing instructor Kellie Montgomery discovers a luxury yacht drifting aimlessly outside a Seattle Marina--with its owners on board dead from gunshots to the head. When Kellie sets out to find their killer, she uncovers a web of secrets that link the victims' sordid past to her own family. Probing the murky depths of murder soon proves far more treacherous than any seas she's ever sailed.