What is the significance of Tom's letter from the merchant marine

Are you sure you mean scene 1? In scene one Tom emerges, dressed as a merchant sailor. In his first speech, he contrasts himself to a magician, giving "truth in the pleasant disguise of illusion," and establishes himself as a poet and the narrator of the play. He tells the audience that the play takes place in the thirties, when there was war in Spain and a different kind of turmoil in America. He warns that the play is a work of memory, and therefore is not realistic.