I found you by Lisa Jewel

Single mom, Alice, finds a man sitting on the beach outside her house. She invites him in out of the rain and discovers that he can’t remember who he is. Twenty-three years earlier Gray and Kristy Ross accompany their parents on their annual trip to Ridinghouse Bay. At the same time Lily, a blissfully happy newly-wed discovers that her husband has gone missing and is determined to find him. All three stories are brought together in this face paced, clever thriller.