The title points to Prospero, the magician of Shakespeare's *The Tempest*, and the poem seems to picture him after his power has gone quiet. The speaker says his "only work is to look," and the line breaks gently shift that idea: "to look" becomes "to look after," so watching turns into caring. The tide is what he tends, though he doesn't command it.
The second stanza makes that humble role clearer. He serves the sea rather than ruling it. He keeps it from noon until night, and what he keeps is mostly an attitude of letting things be. The phrase "I let be" is the heart of the poem. It suggests acceptance, a willingness to give up control.
The mood is calm, patient, and a little wistful. The short lines move like slow waves, and the speaker seems content, even relieved, to be a watcher instead of a master.