Usually clothing made specifically for swimwear, such as bathing suits or bikinis for girls, swim trunks for boys. But really you are capable of getting wet in anything so long as you either have a dry outfit to change into, or don't mind sitting around waiting to dry off.
[[You're welcome to come, Vriska. This is a friendly get together after all~]]
Video;
[[You're welcome to come, Vriska. This is a friendly get together after all~]]