The opening epigraph of When I was Puerto Rican is a metaphor that describes the immigrant experience of people of all nationalities. In settling in a new country, one's identity is not so much changed as expanded, with a new layer added to it. There are the difficulties of adjustment, in learning a new language and customs, but also overcoming the embarrassment that confronts an immigrant who is not readily accepted by the people of the new homeland.
In some sense, Santiago's point is that the ship never really sails because the homeland, one's native country, remains within one even after immigration. The story about the guava plant that begins the memoir is a case in point. If you weren't born "over there," you wouldn't know the proper way of eating it. But in the new land, in New York, Esmeralda passes by the guavas and goes on to the fruits better known in the Yankee culture.
This same cleft of identity, or dual identity, has existed for all Americans apart from the indigenous peoples. Part of the irony in Santiago's memoir is that Puerto Rico is actually part of the United States. But Americans who have originally come from other shores (in other words, most of us) never completely lose their original ethnicity. In some ways, the ship never does sail, although it has sailed. We remain always to some degree what our parents or grandparents or great-grandparents were before they became "American," and this then raises the basic question of what "America" means or is.