"The Idea of Order at Key West" is packed with alliteration, which (along with assonance) gives the poem a lushly musical texture. The poem is largely about sound, and its alliteration often seems to mimic, or otherwise reflect, the sounds it's describing. Take lines 4-5, which describe the sound of the sea:
[...] and yet its mimic motion
Made constant cry, caused constantly a cry,
The /m/ and /c/ alliteration is almost over the top here, evoking the sea's "constant," insistent, plaintive "cry."
In lines 8-11, dense alliteration, together with the speaker's description, underscores the individual "word[s]" that lyrics and poetry are made of:
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.
The language dramatizes the speaker's point: that human language (even when sung or arranged into smooth poetry) can never be as fluid as the sounds of nature, simply because it's broken into words. The /g/ alliteration a couple of lines later—which sounds especially harsh amid all the gentle /s/ and /w/ words and nasal /m/ and /n/ words—evokes the cacophonous side of nature as well:
It may be that in all her phrases stirred
The grinding water and the gasping wind;
Arguably, the poem has it both ways when it comes to its juxtaposition of human song/language with natural sounds. It insists that the two are not "medleyed" together—there's an unbridgeable difference between them—but its descriptions and alliteration keep teasingly narrowing that difference anyway. Listen to how closely the human "self," her "song," and the "sea" are intertwined in lines 37-30:
She was the single artificer of the world
In which she sang. And when she sang, the sea,
Whatever self it had, became the self
That was her song, for she was the maker.
Indeed, throughout the poem, alliteration tends to draw connections between sounds and images rather than distinguishing between them. It links "sky," "sea," and "song," as well as "wind" and "water"—plus the human "we" listening to them. In the end, the poem is richly ambiguous about the relationship between human and natural "voice[s]," or art and reality in general—and it leans heavily on alliteration to capture that ambiguity.