The detail that struck me is me in the second stanza of this poem:

The sea was not a mask. No more was she.
The song and water were not medleyed sound
Even if what she sang was what she heard
Since what she sang was uttered word by word.
It may be that in all she sang there stirred
The grinding water and the gasping wind;
But it was she and not the sea we heard.

Expressing the iambic rhythm in your reading necessitates almost unnaturally heavy stresses on the first syllable of “Even” in line 3 of the stanza and on “may” and “all” in line 5.

Why is this significant? Mainly, I suppose, as a contribution to the hypnotic, incantatory artificiality of the rhythm, which seems to me designed to put to sleep a critical response to what the poet is saying. This rhythm, and the phrasal repetitions make it difficult to hold the thread of an argument that has an air of scrupulous, almost pedantic precision but dissolves into vagueness at every turn. For example, the massive emphasis on “even” suggests someone wanting to concede every possible counter argument before making his claim (emphasising his willingness to do this of course adds rhetorical force to his argument). Logically in the language of Stevens’ time it should be followed by “were” (the subjunctive to express a counter-factual statement or an extremely unlikely possibility) but the indicative “was” virtually concedes that the unlikelihood isn’t really so great. The fifth line segues from the tentativeness of a heavily emphasised “may” to the grand assertiveness of a heavily emphasised “all”. Line six gives a wonderful physical presence and power to the grinding and gasping of water and wind only to be followed by the assertion that what we heard was she and not the grinding sea at all. In what sense or context the assertion is made is quite unclear – whether in the context of the whole situation or more specifically in her song.

Anyone can multiply examples and I daresay that what I’m saying is completely banal. Of course I’m not claiming that Stevens’ pretended logic should be examined as logic and found wanting. He pretends a logical analysis in order to effect the brilliantly precise orchestration of a series of impressions of the paradoxical relationship between the singer and the song on the one hand and the sea on the other, a relationship in which, from the point of view taken in the poem, they’re at once completely separate from and inseparably involved with each other.

I’m sure you can get this feeling without seeing how it is reflected in the metre, but its reflection there adds to the beauty of the poem.