To understand this poem better, it is important to have the background sorted out. Arnold, while still a young, unmarried man, spent time in Switzerland, so this poem is part of his Switzerland series (1847-1850). Publication of Arnold's letters to his friend Arthur Hugh Clough established a consensus among critics and historians that "Marguerite" was real and not imaginary. Arnold most likely changed her name, but she was French and probably Catholic, while he was English and Protestant Anglican. Critics speculate that these two things, culture and religion, may have been the thorns that kept them apart and ended their love "as soon as kindled, cool'd."
The summary of the poem is that in stanza one, Arnold (he himself is the poetic speaker and persona) uses an unspecified apostrophe ("Yes!") to introduce the idea of individual isolation by comparing humans to isles, islands, bounded by "watery shores" all round.
in the sea of life enisled, [...]
The islands feel the enclasping flow,
The dominant metaphor of life as a sea ("the sea of life") carries over to stanza two where Arnold sets up the meeting across the seas between the isolated isles of himself and Marguerite. He provides for their meeting within the metaphor of isolated islands by a "But when" device that allows for the moon's light, sweeping winds, starry nights and the pouring note of nightingales' songs.
But when the moon their hollows lights,
And they are swept by balms of spring,
And in their glens, on starry nights,
The nightingales divinely sing;
And lovely notes, from shore to shore,
Across the sounds and channels pour--
Stanza three exults the chance encouter of these floating, wind swept, song enchanted isles whose margins ("marges"), or boundaries, meet under magical nights. He proclaims the islands to have been united at some distant time: "For surely once, they feel, we were / Parts of a single continent!" Yet, in the same poetic breath, he separates them yet longs for another encounter:
Now round us spreads the watery plain--
Oh might our marges meet again!
Stanza four introduces the question of "Who" ordered the islands be separated and answers that it was Divine Providence that decreed it be so: "A God, a God their severance ruled!" The stanza ends with a return the "sea of life" metaphor saying the sea is once again between the shores of the islands, himself and Marguerite, [This was both metaphorical sea and literal sea as Arnold sailed away from Switzerland back to England.]
And bade betwixt their shores to be
The unplumb'd, salt, estranging sea.