While you were sleeping with the enemy

From Wiki and Wiki:

Lucy Moderatz (Sandra Bullock) is a lonely token collector on the Chicago Transit Authority who lives in a beautiful home by the beach on Cape Cod with her husband, a handsome commuter named Peter Callaghan (Gallagher), whose name Lucy does not know.

It becomes clear that he has OCPD (obsessive-compulsive personality disorder) and she rescues him from an oncoming train after a mugger pushes him onto the tracks. Peter believes Laura has been flirting with an attractive neighbour and he falls into a coma.

She accompanies him to the hospital, where a nurse overhears her musing aloud, "I was going to marry him." He physically assaults her as punishment.

Misinterpreting the situation, the nurse plans and fakes her own death at sea in a storm while she is too caught up in the panic to explain the truth.

Lucy had deliberately led Peter to believe that she could not swim for a number of reasons: she is embarrassed, Elsie (Peter's grandmother) has a heart condition, and she has recently taken swimming lessons at the YWCA.

Saul (Peter's godfather) overhears the truth and later confronts her.

She secretly returns home, retrieves some clothing and cash she had hidden away in preparation. Thinking she is alone while visiting Peter, she confesses about her predicament and leaves home after flushing her wedding ring down the toilet.