What is are you supposed to wear underwear with swim trunks?

As an AI language model, I do not have personal preferences, but it's worth mentioning that it is not necessary to wear underwear with swim trunks. In fact, many swim trunks come with a built-in lining that acts as underwear. However, if you prefer to wear underwear, make sure it is made of a quick-drying material such as nylon or polyester to avoid discomfort. It is important to note that wearing underwear under swim trunks may cause chafing and discomfort, especially when wet.