In this first stanza of the poem, the speaker sets the scene. It is a beautiful day: the sun is "warm," the sky is cloudless, and the ocean's waves dance "fast" and bright. The mood is therefore peaceful and idyllic.
Next, the speaker looks down on the ocean floor and looks at the seaweed. He watches the waves again. He notes the beauty of this scene but also that he is alone. He does not have anybody with whom he can share this idyllic day:
How sweet! did any heart now share in my emotion.
In the third stanza, the tone and mood of the poem changes. The speaker, no longer enraptured by the beauty of nature, begins to focus on his life—specifically on all the things which it lacks, like "hope," and "health," and "peace." To make matters worse, the speaker knows lots of people who have all these qualities in abundance. They call life a "pleasure," while the speaker feels the opposite.
These thoughts make the speaker want to give up. He wants to lie down on the sand and "weep away the life of care." Death would come quickly, but he knows that some people would mourn his demise, just as he mourns the loss of this "sweet day" by thinking such unpleasant thoughts.
But the speaker closes the poem on a final, more positive note: he will not dwell on the negative but let this beautiful day live on in his memory.