Swimwear works hardest when the fabric, fit and care suit how you use it.
Fabrics
Most swimwear is nylon or polyester blended with spandex (elastane) for stretch. Polyester is durable and lightweight but less stretchy, and spandex is not especially durable in chlorinated water, which is why it is usually blended.
Sun protection
UPF compares the burning UV that reaches a fabric with the amount that passes through. A UPF 30 fabric lets roughly 1 in 30 units of UV through.
Wet fabric
Wet fabric can lose up to half its protection, and stretching opens the weave, so a snug suit may protect less than the label suggests. Use sunscreen too, at SPF 30 or higher according to the NHS.
Care and returns
Rinse after swimming and dry in shade. Swimwear with a broken hygiene seal usually cannot be returned.
Sources: Wikipedia: Swimsuit, Wikipedia: UPF and NHS: Sunscreen.
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