The speaker is lying on cool sand in the dark, listening to the ocean, and speaking to someone who loved it. The poem opens in the past tense, with a short list of what that person adored: how the sea moved, smelled, and sang. Then it shifts to "now," and the speaker is alone with the sound.
The poem suggests the person being addressed is gone, though it never says so outright. Instead, the speaker hears the ocean as a full piece of music: waves as the melody, birds as the chorus, wind as the harmony. It's a gentle, almost comforting image, as if the whole shore is performing in this person's place.
The last line is the quietest and most personal. "Remember on sad nights you sang us to sleep" turns the ocean's song into a memory of a human voice. The mood is tender and grieving, but calm. The speaker seems to find the person again in the sound of the sea.