The poem "Descent of Winter: 9/30" is part of a larger group of poems that William Carlos Williams wrote as he returned home to the United States from Europe in 1927. To get a sense of the thesis or theme of this poem—its purpose—it is best to back up and put it into the larger context of Williams's goals as a poet. Williams was an imagist, meaning he wanted his images to carry the meaning and mood of his poems. His poetry is spare, meaning he uses few words, and he seldom explains what they mean, leaving it up to the reader to supply an interpretation.
In this particular poem, Williams does offer a bit of context, supplying this idea: the sea is sad. He then fleshes that concept out with images that guide us in understanding why he made that statement. These images include that the waves are "all broken" and that they are "all the same." The mood of the sea feels the same to him whether the waves are lifting or falling. When he looks at the waves more closely, he sees a "brittle crest" as a wave rises to its highest point, followed by "delicate imperfect foam," then "yellow weed." With the yellow weed he returns to the idea of dull sameness: "One piece [of weed] like another." This sameness of the sea causes him to feel "no hope."
But there is an additional context. The sea makes the speaker feel hopeless because it is like his writing. Speaking to himself, he says,
Your writings are a sea
full of misspellings and
And then, he uses a simile to compare waves to words: "Waves like words, all broken."
When we realize that the waves Williams sees are like his words, we realize why he is sad. He cannot come up with vivid images in his writing at the moment. His poetic inspiration is flat. All his words seem broken and stale, "one ... like another." He is having writer's block. True to form, he conveys his meaning through repeating that what he sees seems all the "same."
After he writes "no hope," there is a dash, indicating a break or long pause. Following this pause, he says,
If not [unless] a coral
island [is] slowly forming
to wait for birds to drop
the seeds will make it habitable.
Having reflected for a moment, he ends on a note of hope. He wonders if what seems like deadness and sameness may be similar to a "coral island slowly forming." If so, he can wait for "seeds" to be dropped. Perhaps, in other words, his lack of inspiration is simply a period in which he needs to wait for inspiration to come. It is not the end of his poetry but a time to be patient, as one is when seeds are first dropped or planted.