SWIMSUITS VS UNDERWEAR: USE IN WATER AND TRANSPARENCY
Here is how to check for lining: take your swimsuit and turn it inside out. Look for the second layer of material under the main layer (it is usually white, but can be any color). Try separating the second layer of the material with your fingers – the lining should easily separate.
Along with lining, recreational swimwear tops can have an underwire for additional support in place of a bra. Certain advanced swimsuits for competitive athletes do not include any padding in the bust area but are made out of thicker material, therefore preventing any accidental transparency.