Your mind and you are our Sargasso Sea, London has swept about you this score years And bright ships have left you this or that in fee: (1-3)
Just over the first line break, the poem jumps from the Sargasso Sea to London. Interestingly, London retains some of the sea's characteristics: it "sweeps" about the woman, bringing "bright ships" to her. The speaker says the woman herself is the Sargasso Sea, but this appears to give her urban surroundings a marine quality as well.
Oh, you are patient, I have seen you sit Hours, where something might have floated up. (11-12)
The woman is described like a fisherman here, patiently waiting with her line in the water in the hopes something will bite. This contrasts strikingly with what's probably her actual situation: sitting in a London living room or salon, hoping some interesting idea or story arises from her conversations and interactions.
Trophies fished up; some curious suggestion; Fact that leads nowhere; and a tale or two, Pregnant with mandrakes, […] (16-18)
Civilized conversation (the kind of conversation two genteel people hanging around London would have) is given very natural and physical qualities here. It can be "fished up" or even "pregnant with mandrakes." Even when the speaker describes a "fact that leads nowhere," we picture a small guppy with "Fact" written on him, swimmingly frantically away from a fishing line.
[…] or something else That might prove useful and yet never proves, That never fits a corner or shows use, Or finds its hour upon the loom of days: The tarnished, gaudy, wonderful old work; (18-22)
The fishing and sea imagery we see in earlier lines now transforms into furniture and decorative objects in a living room. We only ask if something will "fit a corner" or "show use" when we are trying to decorate an interior space, and adjectives like "tarnished" and "gaudy" refer to objects we might find on our grandmother's fireplace. The poem also moves us from thinking about natural to manmade objects – things found on a loom that require labor, or work, to produce.
Idols and ambergris and rare inlays, These are your riches, your great store; (23-24)
"Idols and ambergris and rare inlays" has a nice, melodic ring to it, but it's also a deceptively complex list of objects. While all three probably refer to some kind of manmade object, they also powerfully invoke the natural and spiritual worlds. Idols are manmade representations of deities and spirits; ambergris, often used for perfume, is secreted from whale intestines; and shells, horn, or ivory are often used for decorative inlays.
For all this sea-hoard of deciduous things, Strange woods half sodden, and new brighter stuff: In the slow float of differing light at deep, No! there is nothing! (25-28)
This poem is filled with vague words like "things," "stuff," and "strange," which actually help intertwine the two worlds of the Sargasso Sea and London. We get a lot of descriptive words that lead us toward the sea or the city, but the speaker never commits to saying, "This is happening in the sea, this is happening in the city." We're left in a hazy, in-between space. In these lines, words like "deciduous" and "sodden" make us feel like we're wading through seaweed in the middle of the ocean, but then the phrase "new brighter stuff" brings us back to the idols, rare inlays, and living room decorations of earlier lines.