In the poem "Dover Beach" what sort of action is suggested in the first stanza?

In the opening lines of his poem, Dover Beach, Arnold presents a naturalistic description of the calm and full Dover sea, the image of the moon on the straits, the moonlit shore, the gleaming light on the French coast, and the vast glimmering cliffs of Dover. In this nightscape, the action lies with the moving waves coming on to the shore and going back.

In line 6, 'Come to the window......', the poet suggests a piece of action to his beloved/companion ( presumably his newly-married wife ). The poet asks her to come to the window of the observatory to look at the sea, to hear the sound of the tides, and to enjoy the sweet night air. He further asks her to listen to the waves, to hear 'the grating roar of pebbles'. The pebbles drawn from the beach by the waves, and flung back on to the shore produce a frictional noise, a noise having 'a tremulous cadence slow', a music that the poet urges his companion to take note of : 'the eternal note of sadness'.

The action suggested by Arnold is a contemplation of the destiny of human suffering, a contemplation to be had of the close and intent observation of the sea and the melancholy music that the waves produce in terms of the perpetual to and fro motion of the age-old sea.