CAN I WEAR A SWIMSUIT AS AN UNDERGARMENT?
Although a swimsuit can function as underwear, it is not recommended to use it in that way. Swimsuits can irritate skin and become uncomfortable over a long period of usage. Underwear is made specifically for prolonged wear and is constructed from breathable, moisture-absorbent materials that are pleasant to the touch and comfortable to wear. In addition, underwear is cheaper and will survive longer than a swimsuit worn as underwear.
To summarize, here are the reasons why you shouldn't wear underwear under your swimsuit:
Underwear material is not made to be worn in the pool or any kind of water
It will be ruined by the chemicals in the swimming pool
It will absorb water, making it harder to swim
It will almost always be visible from underneath the swimsuit, which can look unpleasant and unattractive
It can be very uncomfortable
Underwear and street clothes can transport dirt and water-borne disease to the pool, harming other swimmers