This is a neat question because it allows the reader to translate the figurative imagery from the poem into a mental "picture."  SO fun!  Let's use the exact images from the poem to create that picture.  Most of it is from the first stanza:

The sea is calm tonight,
The tide is full, the moon lies fair
Upon the straits:  on the French coast the light
Gleams and is gone; the cliffs of England stand,
Glimmering and vast, out in the tranquil bay,
Come to the window, sweet is the night air!
Only, from the long line of spray

Here the speaker tells us of a calm bay attached to the sea, overspread with the high cliffs of England.  It is evening, and the speaker notices a rocky shore, without much beach (mostly because it's at high tide).  The wide water sparkles in the moonlight for the speaker, and even the night air is sweet. 

From the second stanza, the speaker adds a bit more to the image, but not much more.  He mentions that the beach that can be seen is full of "moon-blanched sand," which to me means white sand reflecting the moonlight.  There is a definite sound here as well as the waves throw the small pebbles around.  The reader can see, even though it is still just the second stanza, that suddenly the images have taken a negative turn.  The irony here is that Dover Beach, even in the second stanza, is still beautiful. It's so surprising to me that such a beautiful image can stir such negative thoughts!