"In the Middle Ages, swimming and outdoor bathing were discouraged in the Christian West and there was little need for a bathing or swimming costume ."
"In classical antiquity swimming and bathing were done naked."
"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."
I'm at Vol.10, I believe -since I read some spoilers- Holo is likely to swim or bathe in later volumes.