Actually the answer can be found at google

"by wearing a swim suit, you reduce the risk of contaminating the pool with waterborne illness and contaminants. Cotton and some other materials can hold detergents, germs, and bacteria in them, which can be released into the water. This can cause swimmers to get sick and even affect the water chemistry and clarity. Cotton also tends to absorb a lot of water and pool chemicals, requiring the pool to use more disinfectant. Additionally, when the cotton absorbs the pool water, it can become pretty heavy, which can weigh down swimmers and make swimming more difficult. Cotton T-shirts also tend not to fit tight to the body, which can increase your risk of underwater entrapment. Finally, cotton breaks down in the water, which clogs the pool filter system. Certain dyed material can also release some of the dye into the pool water, causing poor water clarity and water balance."

There is some other type of swim suit that can cover every part of body u want in the market.

I once saw all the muslims wear t shirt tudung swim in a hotel swimming pool in penang & the water of the swimming pool is so cloudy like mushroom soup. 😳. Should have take photo, so that can increase awareness & see it urself to believe it.

North korean or south korean?

It is due to location. The important keyword for investment is still "location, location & location"