Her towel and robe get magically sent to laundry.

Hermione did a couple of lap of the pool, and stopped at one end. She pushed herself up and sat her nude form at the edge of the pool, staring at the mermaid painting as it flicked her tail and winked at Hermione. The mermaid painting had been in the prefect's bathroom for as long as she could remember, and Hermione wondered how many nude boys and girls it had gotten a look at.

Still dripping wet, Hermione stood up and walked to the other side of the pool to get her towel. As she picked up the basket, she suddenly slipped, and sent the basket flying off to a corner. She broke her fall with an outstretched arm, and slowly pushed herself back up, her body aching from the fall. She walked towards the upturned basket in the corner to retrieve her clothes. When she turned it the right way up, she was in for a shock. "What? Where are my clothes?" Looking around, she suddenly remembered that all clothes kept in the corner of the bathroom would magically be sent down for laundry, and would turn up at her dorm later in the day. So here she was, naked and dripping wet, with no robe or towel to cover herself with.

Hermione folded her arms across her chest, nervously trying to think about what she should do next.