The title of the poem tells us that in the following 30 lines, we're going to get a portrait of a woman. From the first couple lines, however, we can tell that this portrait is not going to be exactly clear or straightforward. The first thing we find out about the "femme"? She (and her mind?) is like the Sargasso Sea. But she's also been living in London for a long time. OK….
This introduction of the Sargasso Sea kicks off a long series of sea-related imagery, all of which seems to be metaphorical for the woman's interactions with other people. The speaker indicates that these people tend to be pretty impressive figures (we're guessing writers, thinkers, or artists), and they always leave something with her, such as ideas, knowledge, or gossip. It's not clear whether the speaker sees this as a good thing, because he points out that the "femme" is the default choice for them and that what they give her is often old and useless.
This giving is reciprocal, as the woman also offers up some equally useless facts, suggestions, and tales. The speaker characterizes these as being decorative, even gaudy, like the ornate vases and statues found in a rich lady's living room.
The poem concludes with the speaker stating that, despite all this clutter of things given to her and things she gives – all these old, useless things and new, gaudy things – ultimately there's nothing that's really the woman's own. And this is the woman's most defining characteristic; this is who she is.
Does this ending make you feel a bit lost at sea? Don't worry, you're not the only one.