So Lucy's grandparents have booked themselves a cruise with their retirement/nursing home. Lucy's whole family thinks this is a bad idea because her grandparents are both becoming senile and get confused easily. And the boat is a standard ocean cruise boat, which is to say that it's HUGE. Lucy volunteers to go with her grandparents and to make sure that they stay safe and don't get lose.
Lucy is shocked at "how far gone" here grandparents are mentally. Her grandfather defecates in his pants almost every day. Sometimes they forget who she is. They can't find their room. She becomes highly stressed and feels like she can't leave them alone for 10 minutes or else she will never find them again. (She is not wrong). But as she cruises with her grandparents she re-reads her grandpa's WWII era memoir and it helps give her perspective on who her grandparents used to be and still are.
So this isn't really a book about a cruise. It's a book about mortality, and relationships and making sure we spend time with our loved ones.