The speaker is looking back at a bad day and comparing it to a much better one. Yesterday was "ugly," today is "fine," and the poem never says why. What it does say is that the change is in how they see: "seeing with / our minds altered." The shift seems to be inside, not outside, though the poem leaves open what caused it. The word "our" suggests the speaker isn't alone in this, and that the feeling is shared.
The last image does the most work. "The bright sea stirring / up and down" is simple and calm, and the line break makes the words seem to rise and fall like the water itself. The sea is moving, but gently, without any sense of threat. After the harsh judgment of the opening, the mood opens out into relief and quiet wonder. The title, "Clarity," fits: it feels like the moment a fog lifts and ordinary things, like light on water, suddenly look vivid and worth noticing.