SWIMSUITS VS UNDERWEAR: USE IN WATER AND TRANSPARENCY
Swimsuits are made with certain types of material that are suitable for water use and can also withstand harsh pool chemicals like chlorine. Additionally, they absorb less water and dry out faster.
Most non-pool water-appropriate undergarments are made from less durable materials such as cotton and can be easily ruined by pool water. Regular underwear will also absorb a significant amount of water, which increases drag and makes swimming more difficult. Extra water retention will also increase the drying time of the garment.
Swimsuits are specifically made to eliminate the need for wearing underwear. Women's swimsuits generally have special lining and additional padding in the bust and crotch areas. Both are added to make the swimsuit more comfortable and to prevent the visibility of one's private areas.