The poem is built on a quiet turn. For four lines, the speaker talks about the sea as if it were a brother, someone who stayed awake late into the night and can now be recognized by his voice. It sounds almost tender, even playful, as though the speaker is describing a sibling they know well. Then the last two lines change everything: this brother's only child died last night at home. The sea, we realize, is a way of hearing a real person's grief.
The speaker doesn't say "I am sad" or describe any crying. Instead, the feeling is carried by the sound of the water, restless and sleepless, and by the plain, almost reporter-like way the news arrives. That restraint makes the loss feel heavier. The mood is hushed and stunned, with a sense of shared sorrow that needs no explanation. The sea's constant, wordless sound becomes the voice of someone who has no words left, and the speaker listens and understands.