Right Across the Bay by Quinn Avery

They were cousins who shared decades of secrets.
And now one of them is dead.
Britta Baxter, beloved fitness influencer and estranged wife of a surgeon, gave up city life to start anew on their grandma's property overlooking Lake Shetek in southwestern Minnesota. After she convinces her cousin, Maxine Thomsen, to revisit the same lake where they'd spent their summers together as kids, Maxine is shocked to learn the boy she loved as a teenager lives across the same quaint bay. Although repairing the decades-old relationship between Maxine and Noah seems possible, a dark ending has already been set in place.
When Britta's body is discovered floating in the bay, Detective Josephine Kelly is called in to assist in the murder investigation. With a near-perfect record, Detective Kelly's sharp instincts rarely lead her astray. Once she senses something is amiss in the neighborhood, she'll apprehend a suspect no one saw coming.
But Britta isn't the only victim on the killer's list, and the body count is about to rise.
Right Across the Bay is an unputdownable thriller fraught with life-changing twists and unfathomable secrets rooted in a shadowy past.