In the crotch are of most men's swim trunks and shorts, there is extra lining and/or padding that works as a substitute for underwear. The lining is usually a mesh material with tiny pores.
However, as technology advances, a growing number of firms are producing swim shorts and trunks without lining. Although there is no need to wear underwear under swim briefs, you may still be at danger of your swimsuit being see-through. Swimsuits are designed with the intention of eliminating the need for undergarments.
So, to conclude, the following are some of the reasons why you shouldn't wear underwear beneath your swimsuit:
The substance of underpants is not designed to be worn in a pool.
The chemicals in the pool will damage it.
It absorb water, making swimming more difficult.
It will often be noticeable from beneath the swimsuit, which might make it appear unsightly and uncomfortable.
Dirt and water-borne illness can be carried into the pool by underwear and street clothing, causing damage to other swimmers.
Special lining and padding in the bust and crotch areas are common in women's swimsuits. Both are included to make the swimsuit more comfortable and to keep one's intimate region's hidden.