Today the idea of swimming nude in a high school gym class seems like something that would absolutely never happen, but in the not to distant past it was the norm for men and boys. The American Public Health Association recommended it in 1926 and schools and YMCA facilities took the recommendation very seriously. Pool chlorination and filtration wasn't as advanced as it is today, and swimwear could cause issues with keeping the water clean.
YMCA's, Schools, and other facilities across the United States adopted the policy of men and boys being required to swim nude- the policies were so prevalent that allowing swimwear was actually the exception to the rule. Boys were required to shower and clean their whole body with soap before entering the pool. The American Public Health Association maintained their recommendation until 1962. Not all schools dropped their nude swimming policies at that time though. Chicago Public Schools, for example kept them for another 20 years.
There's an obvious double standard here. Girls were to wear suits. This was explained away with excuses about girls being more modest.
Can you imagine what would be different if this policy was still in place today? Would more people be comfortable with nudity?