Where this poem takes place is packed with significance. On the surface, what could be more pleasant than the beach, right? But Stevens takes the setting way past sandcastles and starfish, gang.
Think about it: the seashore is a place where two elements come together in a dramatic way. Waves crash on the shore and then retreat back to the sea, and there is a blending of these two separate, but connected, spaces or "zones." These aspects of the setting are reflected in the ideas that are explored in the poem: the connection and blending of the external and internal worlds, the way reality and imagination seem like separate, then mingled, and then again separate elements throughout the poem.
Really, it's all there in the setting! In fact, without the setting, we wouldn't really have much of a poem at all. If Wallace had set this thing at the mall, say, there is a really good chance no one would be reading it today. But he didn't. Instead, he uses the setting to encapsulate everything that the poem is concerned about. Really, it's (dare we say?) genius.