CAN I JUST WEAR UNDERWEAR AS A SWIMSUIT? WHAT ABOUT A T-SHIRT?
Short answer – it depends. In general, it is not advised. Most swimming pools have some type of policy about swimwear usage. A majority of them require you to wear an acceptable swimsuit and prohibit wearing underwear or street clothing.
Those policies are mostly implemented for safety reasons. Clothes worn outside can transport waterborne disease and make other swimmers sick.
Clothing can also transfer dirt and dust particles into the pool, which can throw off the water's chemical balance. Chemicals in the pool act as a disinfectant and help fight germs and prevent the spread of disease. Therefore, it is important to keep dirt from clothing away from the pool.
The rules are also needed to provide some consistency of coverage among swimmers, as well as avoid accidental nudity and unhappy customers. Because underwear usually doesn't have a string to tighten it, the underwear is more likely to slide off your body while swimming. Underwear can also become see-through after getting wet.
At beaches, on the other hand, you can wear whatever you want, making it acceptable to wear bras, boxers, briefs, or anything else you can think of. I have been to some beaches that have regulations about swimwear, but those are very rare.