For Stevens, the poet's job goes beyond putting words together in interesting ways (sorry, Jay Z). He believes the poet has the power to actually complete the natural world, to help all of us experience reality more fully. In some ways, the sea that we experience when we go to the beach is the sea we've been taught to see by writers and artists. And in the context of "The Idea of Order at Key West," the act of writing becomes the act of making the internal world of imagination into an external, concrete reality through the artistic product—whether that's a poem or a song.
Sorry, Wally. Imagination and reality are opposites. Imagination is internal and reality is external. One does not affect the other.
The poem show how our imaginations are influenced by the world around us and how we perceive the world around us is colored by our imaginations. Reality and imagination are intertwined. (Sorry, though—imagining you've already finished your paper on Stevens won't make it a reality.)