Write an essay of that analyze the symbolism/imagery in "The Swimmer." You could go a lot of different directions with this topic, but you might consider relating the symbolism to Neddy as a character and/or to his journey.
Some key areas of symbolism and imagery include: water/pools, swimming, alcohol, changing seasons/weather, youth/aging, and various images of class (think especially of Neddy's fall from the upper class).
If you're having trouble getting started, you might consider examining and analyzing how two (or more) of these types of symbolism/imagery work together or fit together in the story, but you are free to take this topic in your own direction.