Polyester Swim Trunks Polyester is a good material for swim trunks because it's soft, sturdy, and wrinkle-resistant. It dries quickly and retains its shape even after repeated exposure to chlorine and UV rays. Many polyester swim trunks offer UPF 50 for covered areas.
As long as your shorts are synthetic material (typically polyester, sometimes nylon), you should be able to swim in the shorts (and, usually shirt if you want) in most American public and semi-public pools without running afoul with any rules.