The Steel Balls
Captain Queeg's steel balls are the most prominent symbol in the novel. Whenever Queeg becomes frustrated or unsure of himself, he reaches for the balls, which he rolls incessantly in his hand. Since Queeg strives for perfection and control, but rarely achieves it, the balls can be seen as a symbol of his neutered masculinity. During the trial, one of the Navy psychiatrists suggests that the balls could be "an expression of rage and hostility toward the world."
The Yellow Stain
Literally, the yellow stain is created when a dye marker is dropped overboard by the Caine during a military exercise. Since the marker is dropped prematurely, and the Caine hastily retreats before the mission is fulfilled, the marker comes to represent Queeg's cowardice. It is, in essence, his yellow streak and is the basis of his nickname, Old Yellowstain.
The USS Caine
The USS Caine is a strong symbol within the novel. Literally, the ship is a destroyer that has been refitted as a minesweeper. In many ways the ship is a "destroyer": it effectively destroys the character of Queeg and the career of Maryk, in addition to inflicting harm on many of the men aboard. As a mine sweeper, its duty is to bring potentially deadly ordinance to the surface and to destroy it. Thus, the ship's duty reflects Maryk's relief of Queeg, a psychological explosive. Tom Keefer comments on the similarity between the Caine and the biblical Cain; like the biblical Cain, the men aboard the ship are outcasts. Of course, Cain's murder of his brother can be viewed as a parallel to Maryk's eventual character assassination of Queeg, his brother seaman.
As part of his court-martial victory dinner, Keefer has a cake prepared with the title of his book, Multitudes, Multitudes, written in thick yellow icing. The yellow icing suggests that the novel, a fictionalized representation of the war, is as much a cowardly act as Keefer's behavior in the courtroom or Queeg's behavior during the yellow dye and gun battery incidents. The cake also represents the saccharine, fictional view of life Americans prefer, in contrast to the stark and less tasteful reality of the world.
Kamikazes were Japanese suicide bombers who directed their planes into enemy ships. The kamikaze incident on the Caine is a turning point for Willie; immediately following the incident he writes his proposal letter to May. As the plane strikes, Willie calls the pilot a "fool" and experiences "an overpowering tearing regret at not having married May." In some sense, Willie may see the event as a reflection of his own life, believing that without May he too is on a course of self-destruction.
Willie's music is symbolic of the sophomoric approach to life he adopts throughout the first half of the novel. He makes some money playing piano and performing original compositions in nightclubs, his most lauded song being "If You Knew What the Gnu Knew," a piece comparing animal and human love-making. Music also represents the easy life he has led. One sign that he is maturing is when he attempts to reject the admiral's offer to remain in Hawaii so that he may entertain at the admiral's parties, instead desiring to move to the Caine and enter active duty as soon as possible.