Underwear-Swimwear Combinations

[Torchwatch]

Good underwear is absorbent, it sucks up liquid water then wicks away water vapour leaving you dry and comfortable but holding onto stains keeping your outer clothing clean and odour free.
Underwear worn while swimming in water will become saturated and saggy and once you come out of the water will remain wet for ages, not a good look.

Good swimwear doesn't absorb water, it allows water to pass through the weave or knit so that a swimsuit will drain as you leave the water remaining only damp. Worn as underwear swim briefs will tend to feel a little damp and sweaty.

Sharkskin swimwear does not get wet even while swimming, it is very expensive, fits tight to the body and moves almost without friction through water. Worn as underwear they would trap all your sweat and you'd probably go all wrinkly if worn all day under clothes.

For one garment to swim and use as underwear I'd go for a Nylon or Lycra swim brief with an absorbent inner lining at the front. There's plenty of choice in brands, colours and styles depending on your personal tastes.