The sea is everywhere in "Dover Beach." It shows up in different places and in different forms, but we feel its power all over the place. Sometimes it's a physical location, something you can actually see, like the English Channel or the Aegean Sea, and sometimes it morphs into a metaphor for the fate of humanity. Heavy stuff, for sure.
Line 1: This is a really simple opening line, focused on a really simple image. We think that's a key part of this poem's effect. We start with this calm, vivid picture in our heads, and from there everything slowly dissolves away. Over 37 lines, the speaker strips away our illusions, and shows us the nightmare behind this calm ocean.
Line 8: Another powerful image, this time of the sea meeting the land. It's important to notice how much time Arnold spends making us really see this vision of the coast of England in the moonlight. The sea is going to turn into a huge metaphor in this poem (so stay tuned), but for now it's just a pretty spot.
Line 16: In this line, the sea is part of an historical allusion. The speaker uses the sea (in this case the Aegean, which is part of the Mediterranean) to connect him to the ancient playwright Sophocles. In this passage the sound of the rising and falling tide is used as an analogy for the "ebb and flow of human misery" (line 17).
Line 21: This is one of the major, go-for-broke metaphors in "Dover Beach." The speaker uses the idea of the sea that he's spent so much time building up, but this time he turns it into a metaphor for the human belief in a higher power. The real sea of the English Channel is reimagined as a "Sea of Faith."