Answer: it emphasizes the displacement of the speaker's family.
Throughout the poem, the author maintains an extended metaphor about swimming. As she tells us the story of how her family escaped the island, she talks to us about swimming, drowning, the beach, the pool, etc. This is intended to emphasize the displacement of the family. Alvarez feels lost and adrift, much like we would feel if we were lost at sea.
https://www.sparknotes.com/lit/commonsense/section3/
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