All The Lies
Amy's marriage is the kind you plan an escape from rather than leave. Her husband is controlling and violent, and she has been managing him for years without anyone outside the house noticing.
On a short cruise she meets Liz, a younger woman who happens to look a good deal like her, and they spend enough time together to be taken for friends. Then there is an accident, and a great many people die. Amy comes round in hospital to find that a mix-up over personal effects has everyone treating her as Liz. Liz is on the missing list, presumed dead.
Nicola Sanders builds the book on the decision Amy makes in that hospital bed: correct the record and be returned to her husband, or say nothing and take a stranger's life for as long as it holds. It does not hold cleanly. Liz turns out to have had a life of her own to step into, with people in it who knew her, and questions about the accident that Amy is now the only person positioned to answer.
A woman in an abusive marriage who is mistaken for another passenger after a fatal accident and lets the mistake stand.
A young woman Amy befriends on a cruise, who resembles her and goes missing after the accident.