Get a polyester-spandex blend with UPF 50+ rating.

Polyester-spandex blends.

Cotton or cotton blends are useless for sun protection — they lose UPF when wet and stretch out. Polyester holds its UPF rating even after hours in the water and chlorine. Spandex gives the stretch so it fits snugly (loose fabric lets UV through).

Avoid nylon — it degrades faster in sun and chlorine. The UPF label matters more than the brand. Any material rated UPF 50+ blocks 98% of UV rays. But synthetics like polyester last longer than cotton or rayon.

A good one costs $15-25 and lasts a couple seasons. That’s cheap peace of mind.