I raced through this, and loved the dynamics between the characters in the lifeboat - their petty squabbles, the women coming to the fore - but then something happened (or rather didn't happen) in the final third of the book, which is mostly set in a courtroom, and is mostly reported rather than showing the action. It rather trailed away, and all the tension that Rogan had built dissipated when we learned nothing new.
Some people have written about how Grace is an unreliable narrator, but it seemed to me that her account of events pretty much tallied with Mrs Grant's and Hannah's. I wondered if Grace was supposed to have

Spoiler

killed Maryanne, who says that she will tell people that Grace's husband bribed her way onto the lifeboat, and that Grace did kill Mr Hardie

but this is dealt with in such a slight way it barely registers.