The Pisces by Melissa Broder

Lucy has been writing her dissertation for nine years when she and her boyfriend have a dramatic break up. After she hits rock bottom, her sister in Los Angeles insists that Lucy dog-sit for the summer. Staying in a gorgeous house on Venice Beach, Lucy can find little relief from her anxiety - not in the Greek chorus of women in her love addiction therapy group, not in her frequent Tinder excursions, not even in Dominic the dog's easy affection.

Everything changes when she becomes entranced by an eerily attractive swimmer while sitting alone on the beach rocks one night. But when Lucy learns the truth about his identity, their relationship, and Lucy's understanding of what love should look like, take a very unexpected turn.