"Portrait d'une Femme" moves between two settings: the Sargasso Sea and London. It alternates between chaotic, natural sea imagery and depictions of civilized urban life. Interestingly, both the sea and the city seem to be filled with rubbish, whether this takes the form of "Strange woods half sodden" (26) or "tarnished, gaudy, wonderful old work" (22). Somehow both the natural world and the manmade world associated with the woman become collection bins for debris and useless objects.
The speaker's association of the woman with the sea is part of a sexist tradition that associates women with the physical and natural (e.g., "Mother Nature"), while men are the civilized thinkers.
Although we intuitively think of the sea as part of the natural world, the poem actually depicts the sea as a place for commerce and cargo. The poem therefore suggests that the contrast between London and the Sargasso Sea is not so large.