Whereupon Mary...spirals, hunches her shoulders, digs her elbows into her ribs, holds out two quivering forearms on either side of her, takes in breath but makes no other sound nor any other movement to relieve her situation (never having encountered it before) freezes stock-still and wide-mouthed while something squirms, twists, writhes inside her knickers and finally (because eels are adept at extricating themselves even from the most unlikely predicaments) squeezes itself out by way of a thigh-band, flops to the grass and with unimpaired instinct snakes toward the Lode. (Waterland, chapter 24)
In a novel full of meditations on sexuality and its effects, this scene is one of the most important. In it Mary, Freddie, Tom, and Dick flirt with their sexual identities--Mary, Tom, and Freddie as children, and Dick as an adult. Their self-discovery and curiosity start a chain of events that leads to Freddie's death, Mary's abortion, Dick's death, and Mary's insanity.
Symbolism as well as explicit description make up the first strands in Swift's web of desire and intrigue. For Freddie to shove an eel into Mary's knickers is fairly crude in Freudian terms, but coming as it does at the end of a scene of growing sexual tension, it is both a climax and a completion. In Great Expectations there is a similar scene rife with symbolism. Miss Havisham's wedding dress catches ablaze, and Pip smothers the fire with his body, burning himself in a strange climax and consummation of Miss Havisham's nonexistent marriage. Miss Havisham consequently dies.
In both of these, sexuality leads eventually to death and ruin--the ancient link between reproduction and destruction. Dickens, in his (self-proclaimedly, at least) morally upright Victorian age, was not as free to use sexual imagery as Swift is in his modern work. Yet the two, as bildungsromans--concerned with the maturations of their main characters--explore these themes of the double peril of adulthood /sexuality and death.