"Cliffedge" begins with the question: "What is it about the sea that summons people to it?" (p.149) With this opening, Graham Swift's primary thematic device, water, is called into play. Water is the controlling element in this short story, a story about the narrator, his "potato-head" brother Neil, and the narrator's marriage.
Neil represents a type of character that appears frequently in Swift's work, that of the idiot child, frequently retarded and always a problem for his siblings and parents. Neil is described in terms of the sea. Although the narrator and his father easily become seasick, and experience what is described as "fear of the waves" (p.151), Neil requests to go out on a fishing boat and catches many fish when his father and brother catch none. Neil is not afraid of the water and would not describe it as his brother the narrator does -- as a "sleeping monster" (p.149).
Each year the narrator's family takes a trip to "Cliffedge," not its actual name, but a name given to the place by the narrator. The tall white cliffs contrasted against Neil's mental illness conjure up images from King Lear, and the danger is similar, as Neil seems driven to throw himself from the cliffs down into the sea. Whenever the narrator and his brother take walks (and Neil always wants to walk along the cliffs) the danger is palpable, and makes the narrator quite nervous.
The narrator's wife resents Neil, and seems to pity the narrator. She is never actually present in the story, for the entire tale takes place at Cliffedge and she does not accompany the two men on their vacation. The narrator suspects that she takes a separate vacation with her lover. The narrator has tried to find out more about this lover, but has not been able to get any specific information. The fact that his wife never speaks of her lover and that he never asks follows another one of the philosophies in Swift's writing, that what you do not know cannot really affect you. With this reasoning, it is best to ignore what is hidden from view.
The story ends with one of the narrator's greatest fears coming true. While his back is turned Neil manages to jump over the cliff's edge. Although his death is fairly certain the narrator refuses to accept it, and goes back to Cliffedge every year hoping that Neil will return. He dreams at night that he is "alone in the boat. I am leaning over the side looking at my line disappearing into the water. I know that Neil is somewhere there in the depths and I will catch him." (p.157) In quite literal terms, Neil has become a part of the sea, a part of the water that haunts Swift's writing.