What is the poem "Dover Beach" about?

Using vivid images of the sea to do so, Matthew Arnold writes his poem about a world bereft of all beauty, truth, and optimism.  In fact, there is only one positive stanza in this poem:  the first one.  This first discusses the positive images of the ocean where the “sea is calm” and the “moon lies fair” and the “tide is full.”  This is where the happy images end, however, because the second stanza refers to all of the negative sea images with its “moon-blanched land” and its “grating roar” and its “tremulous cadence.”  The negativity continues with the third stanza where the negative aspect of the sea even pervades the past, specifically during Sophocles time where it continued to affect the “turbid ebb and flow / Of human misery.” 

Of course, the importance of the poem stands in the two metaphorical stanzas at the end.  Stanza four, that focuses on the "Sea of Faith" is very intent on its pessimism.  Where there was faith in the speaker's mind, now there is nothing.

Now I only hear / Its melancholy, long, withdrawing roar, / Retreating, to the breath / Of the night wind, down the vast edges dream / And naked shingles of the world.

Finally, as the speaker begs for lovers to remain true, sadness stands paramount yet again in one of the bleakest statements in the poem.  The world that once seemed beautiful and new is now bereft of beauty and happiness.

[The world] hath really neither joy, nor love, nor light, / Nor certitude, nor peace, nor help for pain; / And we are here as on a darkling plain / Swept with confused alarms of struggle and flight, / Where ignorant armies clash by night.

"Dover Beach" is obviously not a poem to cultivate happiness within the reader.  What is interesting, however, is Arnold’s ability to allow the sea images to pervade his poem whether it is in the midst of happiness (of the first stanza) or of misery (of the rest of the poem).  Arnold is certainly a master of imagery!