What is importance of The cruise ship?

The cruise ship is a symbol of freedom and exploration, especially for Grandma. Through much of the first half of the novel, Grandma is morose and does not talk much to anyone, struggling to make friends at her new home after Grandpa's passing. However, onboard the cruise, she finds herself reconnecting with certain passions. At the end of the novel, she tells Iris she intends to live on the cruise ship full time, citing a quote from Moby-Dick about how the best places are those you cannot find on a map.