Laughter, accompanied by shouts and splashing water, echoed along the corridor. Kiri, who'd been about to slouch off up another staircase on the long journey to the common room , stopped and turned around. This part of the castle was supposed to be flooded now, wasn't it? Not that the sounds she heard sounded much like Grindylows or anything similar. They were more like something she'd hear at a crowded pool.

She decided to investigate. Turning around a corner, she saw... well, it was like a crowded pool, really. The corridor was flooded, but a big group of students (and one or two teachers as well, apparently) were enjoying themselves in the water. Most of them were even wearing swimsuits. (Some of those could only technically be called swimsuits. If their makers had used any less fabric, they needn't have bothered at all.)
It looked... fun! No one seemed about to drown, either, so it had to be safe, too. Kiri wasn't much of a swimmer, especially now; she'd thought that if she never had to look at water again in her life, it still would put her off swimming, but she found herself wondering whether to join in.
Not that she had a bathing costume. Hers had been lost months ago with most of her possessions when the Slytherin common room had been flooded.

Nonetheless, she dropped her bag and struggled out of her shoes. There was a heap of clothes a little way away, out of reach of any water. She put her bag there along with her shoes and her black school robes. Then she pitter-pattered closer on naked feet (the stone floor was a little cold) until she stood close to the water, watching the other students swim and bounce about, a little indecisive and uncertain of what to do.