Beach volleyball is more common in areas close to the beach. Beach and indoor players are also significantly taller than the average person-- in the league that my college was a part of, the shortest woman playing middle blocker was still 185cm. Height and beach access both have correlations to socioeconomic factors.

For various reasons, the US has huge racial disparities when it comes to swimming ability. Most African American and Hispanic children don't know how to swim.

As others have mentioned, football, basketball and to a lesser extent soccer/baseball tend to be the "pleb" sports. But even within a sport, you'll see class and racial distinctions when you compare different positions. When you look at pros and college, the majority of quarterbacks and centers are white. QBs also tend to come from wealthier families. Meanwhile, wide receivers and running backs are mostly black. The defense is mostly black, and I don't think there's a single non-black cornerback in the NFL (unless you miscount Tre'Davious White)

Out of curiosity, how common is it for public elementary, middle and high-schools to include swimming lessons in (mandatory) PE class in general? How often do schools without their own swimming pools take their PE class at a nearby public pool?

This works well as a heuristic because visible racial characteristics are visible and the rich don't usually want to play with the poor.