The Survivors by Jane Harper

Kieran Elliott's life changed forever on a single day when a reckless mistake led to devastating consequences. The guilt that haunts him still resurfaces during a visit with his young family to the small coastal town he once called home.
Kieran's parents are struggling in a community which is bound, for better or worse, to the sea that is both a lifeline and a threat. Between them all is his absent brother Finn.
When a body is discovered on the beach, long-held secrets threaten to emerge in the murder investigation that follows. A sunken wreck, a missing girl, and questions that have never washed away...