In just three short lines, Dave Bonta sets a calm morning scene and then gives it an odd twist. The sea at high tide is full, loud, and in constant motion, and the speaker describes that noise as "the din of her business." By calling the sea "her," he turns it into a woman hard at work, bustling around with her errands and affairs, and the water's roar becomes something like busy chatter or the clatter of a shop opening for the day.
The speaker doesn't say how he feels outright, but the word "din" suggests the sound is loud and a little overwhelming. There's a hint of wry humor, too, as if he's being gently shut out by someone who has far more important things to do. The mood is lively but not hostile: a morning that is awake, crowded with sound, and indifferent to the person listening. The sea carries on with its business, and he simply has to stand there and hear it.