There was a death in Max's early life. Who was it, when was it, and what were the circumstances surrounding their death?

In the novel The Sea, by John Banville, we follow the life of Max Morden, an old man who returns to the seaside town where he spent a summer fifty years earlier. His wife has recently died, and he is revisited by memories of an earlier death that he experienced during the summer that he was there. During that earlier time, he became acquainted with a family, the Graces, who he was infatuated and a little obsessed with. He thought that he was in love with the mother, Connie, but he also had a strange relationship with her daughter, Chloe, and became friends with Chloe's twin, Myles. Throughout their friendship, Chloe made it known that she was attracted to Max, kissing him flagrantly and taking him by surprise. Eventually, he overheard some words between Rose, the nanny, and Mrs. Grace that he thought mean that Rose and Mr. Grace were having an affair. Later, Rose and Max watched helplessly from the shore as the twins drowned in the sea. Max had told them that he believed Rose and Mr. Grace were having an affair. Being twins, Chloe and Myles did everything together; they were inseparable. So it follows that they would have been together when they died. It is a very symbolic incident for Max and one that defines his childhood, because it was a summer in which he "came of age" in more ways than one and learned that all actions have consequences.