When reading House of Mirth, keep an eye out for words like "flood," "water," "drown," "submerge," "sink," "float," and "ocean." You'll find a slew of references to drowning and floods and the like, starting with this passage right here:
[Lily] knew that she hated dinginess as much as her mother had hated it, and to her last breath she meant to fight against it, dragging herself up again and again above its flood till she gained the bright pinnacles of success which presented such a slippery surface to her clutch. (1.3.73)
Think of the image of drowning in a great flood of water as a sort of despair. In Lily's "Character Analysis," we talk about her transformation throughout the course of the novel from a material girl to a moral woman. This passage – from Part I, Chapter Three of the novel – gives us a glimpse into Lily's character back when she's still relatively shallow and amoral. In her mind, the greatest despair – the feeling of drowning – has to do with poverty. To Lily, the flood of water is "dinginess."
Now look at this passage, at the moment of Lily's moral apotheosis (after Gus Trenor propositions her):
Over and over her the sea of humiliation broke – wave crashing on wave so close that the moral shame was one with the physical dread. […] His touch was a shock to her drowning consciousness. (1.13.67)
Lily is despairing once again, and once again that despair is described as drowning in a flood of violent waters. Except, this time, it has nothing to do with money. Lily is feeling a flood of shame and immorality, not of "dinginess." Why? Because she's learned to recognize immorality when she sees it. Earlier in the novel, "undisturbed by scruples," morality wasn't on Lily's radar. This scene, at the end of Book I, constitutes a moral awakening. It's a big moment for Lily and the turning point for her transformation.
Interestingly, Lily isn't the only one spending some quality time with the novel's most intense imagery. When Gerty Farish realizes that Selden is in love with Lily, not her, it goes a little something like this:
It was all as meaningless as the boom of waves in a drowning head, and she felt, as the drowning may feel, that to sink would be nothing beside the pain of struggling to keep up. (1.14.25)
And, shortly after, we have:
Alone with her cousin's kiss, Gerty stared upon her thoughts. He had kissed her before – but not with another woman on his lips. If he had spared her that she could have drowned quietly, welcoming the dark flood as it submerged her. But now the flood was shot through with glory, and it was harder to drown at sunrise than in darkness. (1.14.59)
When Lily comes by, distraught and frantic later that night, "She [lays] both hands on Gerty's shoulders, with a smile that [is] like sunrise on a sea strewn with wreckage" (1.14.92). That's a lot of ocean imagery in about ten pages of text. What's going on? It looks like, for Gerty, this drowning imagery is a question of willful submission as opposed to violent struggle. Should she give in to the flood (that is, give up all hope of having Lawrence for herself) or fight against it (battle Lily for the man-turf)? Because Gerty chooses to help Lily and give up Selden in the process, it's as though she chooses to drown. When Lily comes by "like a sunrise on a sea strewn with wreckage," it's because Gerty gave up, sank beneath the waves, and is now the flotsam and jetsam floating around in the aftermath of the storm.
How does Selden's character make use this water imagery? Check this out, from the same stormy Part I, Chapter Fourteen:
Well, he had strength for both – it was her weakness which had put the strength in him. It was not, alas, a clean rush of waves they had to win through, but a clogging morass of old associations and habits, and for the moment its vapours were in his throat. But he would see clearer, breathe freer in her presence: she was at once the dead weight at his breast and the spar which should float them to safety. (1.14.44)
Remember that Selden wishes to take Lily "beyond – beyond the ugliness, the pettiness, the attrition and corrosion of the soul." This process of dragging her up and away is, in his mind, akin to making their way out of a muddy bog. Just like Lily, the imagery is very much tied to morality.
Now, let's jump to the end of the novel, right before Lily's death.
That was the feeling which possessed her now – the feeling of being something rootless and ephemeral, mere spin-drift of the whirling surface of existence, without anything to which the poor little tentacles of self could cling before the awful flood submerged them.
Remember, Lily has conquered a moral crisis already. Immorality attacked her like a rush of waves, but Selden and she managed to pull themselves up and out of the water. Now, a different kind of flood hits Lily, this time in the form of her on-going identity crisis. Lily believes herself a "cog without a machine" since she's dropped out of society. In the "Character Analyses" we talk about social determinism, the idea that Lily is suited for society and incapable of living outside it. Because of this, Lily feels useless and rootless as a working class gal. Her "tentacles of self" have no footing; she's in a flood of her own uselessness.
The very last occurrence of water imagery comes in the final chapter, when Selden finds Lily dead: "The bitter waters of life surged high about him, their sterile taste was on his lips" (2.14.32). Selden may have pulled Lily out of the "morass" (or bog) of immorality, but he couldn't save her from what he considers "the bitter waters of life." Factors like social determinism or the gilded cage of society mean that Lily is never really free – or in this case, never safely on dry land.