Everblue (Mer Tales #1) by Brenda Pandos

Best friends share everything with each other. Or do they? Seventeen-year-old Ashlyn Frances Lanski is tired of her boring, single life. Spending time with her best friend Tatiana, dreaming about kissing Tatiana's twin brother Fin, and swimming competitively are her only sanctuary. The girls plan to leave their drab lakeside town far behind for college. But when Tatchi fails to return home after a family emergency, and no one knows where the family has gone, Ash chooses to do something drastic to find them.

Ashlyn is about to discover what she’d thought to be true her whole life, wasn’t, and the truth, too fantastical to imagine. Secrets lurk beneath the deep blue waters of Lake Tahoe, secrets that will change Ashlyn's life forever.