You might like to consider the way in which the central quest of the swimming journey that Neddy Merrill undertakes is linked to the cycle of life and his own failing physical condition. He is a character who is associated with youth and vitality, although we are told he is not the youngest of individuals:

He was a slender man--he seemed to have the especial slenderness of youth--and while he was far from young he had slid down his banister that morning and given the bronze backside of Aphrodite on the hall table a smack, as he jogged toward the smell of coffee in his dining room. He might have been compared to a summer's day, particularly the last hours of one, and while he lacked a tennis racket or a sail bag the impression was definitely one of youth, sport, and clement weather.

However, as his own personal odyssey that he sets himself develops, the ease with which he starts to swim slowly disappears, and he begins to realise that he might have taken too much on:

He was cold and he was tired and the naked Hallorans and their dark water had depressed him. The swim was too much for his strength, but how could he have guessed this, sliding down the banister that morning and sitting in the Westerhazys' sun? His arms were lame. His legs felt rubbery and ached at the joints. The worst of it was the cold in his bones and the feeling that he might never be warm again. Leaves were falling down around him and he smelled wood smoke on the wind. Who would be burnign wood at this time of year?

Note the way that the protagonist's own sense of physical decline is mirrored by the swift change in seasons, as the height of summer has been exchanged for the depths of Fall and the encroaching winter as Neddy is forced to realise his own approaching death and sense of mortality. The image we are given of a weak Neddy at the end, looking into his empty house and lost and confused and isolated about what has happened, is very different from the image we are given him at the beginning, and it is clear that the symbol of his swimming quest severely questions the worth of his life and makes us feel sympathetic towards him.