Here's an extract from a superbly provocative little book, now alas out of print, called Literature and Criticism , by H. Coombes (Penguin, 1963; 1st published in England in 1953). It's from the chapter on imagery.
The Sea of Faith. . . the imagery here is not really [very] precise and illuminating. Something which once flourished and now is fading is not rightly comparable with the sea at all: the poet's excess of melancholy apparently allows him to conceive of the sea as now ebbing only and as not subject to a recompensing flow. And the simile of the third line belittles and not enhances the idea of a great "Sea of Faith" by comparing it to something very ordinarily tangible: "the fold of a bright girdle," especially when "furl'd", do nothing to suggest the sea's wide and enveloping yet scattered character. "Bright girdle" seems to be confusedly connected in the poet's mind with the idea of a comforting and protective cloak; which the cold sea is not. The phrase is only a convenient emblem to contrast with the poet's present gloom and so to make it appear the greyer. The last five lines have the impressiveness of its kind of romantic poetry: a certain sonorous music and a vague-mysterious picture and atmosphere. But these qualities are there for emotive purposes only: Arnold is indulging in his sadness and he intends us to indulge too; he frankly describes a scene in such a way as to lull us into accepting it (does it competently too, if without great distinction), forgetting that it is a "Sea of Faith" he is dealing with. The actual sea's sounds and shores engross him, and the image has in fact vanished; and the result is strange: for instead of deploring the disappearance of Faith, which was really Arnold's theme, we find ourselves enjoying a description of the sea. The failure of the imagination and intellect to create and control effective imagine is common in Victorian poetry generally.
Was once, too, at the full, and round earth's shore
Lay like the folds of a bright girdle furl'd.
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.