Cold Blooded Liar by Karen Rose

Sam Reeves is a kindhearted psychologist who treats court-ordered clients. After one of his patients--a pathological liar--starts revealing plausible new details from a long-unsolved serial murder case, he's compelled to report anonymously to the SDPD tip line, though his attempts to respect patient confidentiality land him facedown and cuffed by the aggressive (and cute) Detective McKittrick.

San Diego homicide detective Kit McKittrick loves the water. She lives on a boat, and when she's not solving crimes with the SDPD, she's assisting her foster sister with her charter fishing business, scuba diving, or playing with her poodle. But there's nothing that intrigues Kit more than a cold case, so when an anonymous caller leads her on the path of a wanted killer, she's determined to end the decade-long manhunt.

Sam is soon released but goes home with both a newfound distaste for the SDPD and a resolve—not unlike Kit's—to uncover the truth. Kit and Sam repeatedly butt heads in their separate investigations but are forced to work together to find one of the deadliest serial killers the city has faced in a decade.