Two short lines hold a surprisingly full picture. A woman, the "her" of the poem, has spent the whole morning with something in her hands, and the poem describes it as "a sea with one fish." The title, "Salt," nudges us toward a likely reading: she may be carrying a bowl or pan of salt water, or perhaps the salt itself, with a single fish moving inside it. That is an inference, since the poem never says exactly what she holds.
What the speaker feels is quiet wonder. Nothing is explained or judged. The image is simply offered, as if the speaker watched her and found the sight remarkable. The phrase "all morning" suggests patience, a slow, steady attention. Her hands become small enough to hold a sea, and the sea becomes small enough to be held.
The mood is calm, delicate, and a little mysterious. The single fish adds a touch of loneliness, but also tenderness, because it is cared for.