In Moby Dick by Herman Melville, there are motifs, symbols and themes. The sea is a motif that runs throughout the story. A motif is defined as a recurring idea or image that is a single word or short phrase that reinforces a writer's theme(s). The sea motif in Moby Dick represents the depths of human knowledge that are beyond comprehension and supports the theme of the knowledge bound by limits.
Queequeg’s coffin is a symbol that alternately stands for varying aspects of life. A symbol is defined as an image that represents an idea. It is important to note that a symbol does not represent an object; it represents a complex idea that is benefited by being reduced to a memorable and more easily understood image. The idea that the coffin represents is life and it's phases. First it represents the death of life as Queequeg has it made for his impending death. Then it represents the fullness of life when, after he doesn't die, he uses it to store his possessions and to be the receptacle of the stories his tattoos tell. Finally it represents the renewal or survival of life when it becomes Ishmael's life raft when the Pequod sinks, saving him and the story of Ahab and the Great White Whale.
The doubloon is part of the theme related to the vagaries of fate: fate is fickle. A theme is defined as the idea, philosophy, moral or concept that the author wants to teach the reader through the literary work. A theme usually needs to be conveyed in a whole sentence as opposed to a motif, which is conveyed by a single word or a short phase. Different characters find different meaning in the doubloons when they are examined as the indicators of fate. This signifies Melville's theme that people sees what they want to believe when they try to interpret and understand the auguries of fate, like signs, omens, and portents.