Swimsuits Parade, Sydney, Australia

History
In Classical antiquity swimming and bathing was done nude. There are Roman murals which show women playing sports and exercising wearing two-piece suits covering the areas around their breasts and hips in a fashion remarkably similar to a bikini of ca. 1960. However, there is no evidence that this was used for swimming. All classical pictures of swimming show nude swimmers.
In various cultural traditions one swims, if not in the nude, in a version in suitable material of a garment or undergarment commonly worn on land, e.g. a loincloth such as the Japanese man's fundoshi.