The Boathouse.

When an emotionally-fragile young woman takes a job as nanny to two troubled children at a remote summer cottage, she falls in love with the children's father, while becoming enmeshed in the mystery of their estranged mother - with whom, it turns out, the young woman has her own fraught history. As the summer progresses, she begins to suspect that the family has a dark history that they are desperate to keep secret.