Hello, Old Sport
The sport a person plays can say a lot about that person. Someone who is very flexible might get into gymnastics. Someone who doesn't mind getting aggressive might like football. And someone who trips over his or her own feet stays home and plays Mario Tennis. Not that we're speaking from personal experience…
The sports the Jarretts enjoy speak volumes. They exclusively play sports associated with rich country clubs, sports like golf and swimming. Beth doesn't have a job; she just plays golf all day and complains when it's cold outside. Her attitude illustrates both her general demeanor and the family's economic status.
Conrad is on the swim team, which is a little more symbolic, considering the fact that his brother drowned. When Conrad returns to school, he has lost a lot of weight from being in the hospital. But he swears it won't affect his ability to be on the team. "You don't have to be heavy to swim" (2.41), which is a line dripping with double meaning if we ever heard one.
Conrad is light in weight but heavy in mind. There's a lot weighting on him, and he eventually quits the swim team. All his friends on the team were also his brother's friends, and the reminder of his brother's death weighs him down.
The book doesn't overwork this metaphor. Conrad doesn't triumphantly return to the swim team or anything like that. In fact, he's happy he quit. He gave it a shot, but he never actually liked it. It's one of the rare times he recognizes his emotions and acts on them.