In "Dover Beach," consider the tides here and how the speaker dwells primarily on the ebb tide. What does that seem to say to him about his world?

In the opening stanza of Arnold's poem, the sea is 'calm', it being 'full'. Later in the same stanza, the poet gives us the image of the endless backward and forward movement of the waves, 'the grating roar of pebbles' generating, 'with tremulous cadence slow', a melancholy music eternally audible.

Arnold dwells on the ebb tide in the 3rd stanza:

The Sea of Faith

Was once, too, at the full, and round earth's shore

Lay like the folds of a bright girdle furled.

But now I only hear

Its melancholy, long, withdrawing roar,

Retreating to the breath

Of the night wind, down the vast edges drear

And naked shingles of the world.

These lines refer to an allegorical 'Sea of Faith' which was, in the past, a mode of protection for the whole of mankind, like a rounded piece of clothing around the shores of life. That faith of the pre-Industrial days is now gone in the modern times of doubts, disputes, conflicts & controversies. The receding movement of the 'Sea of Faith' leaves back a barren, waste land, the world lying stripped naked. The image of the ebb tide thus signifies the crumbling of faith and its resultant spiritual vacuum.