Door opens.

Hermione was just getting to the apex of her session when she heard the alarming sound of the door opening.

Terrified she realized her clothes and towel were to far away and when she heard foot steps entering she did the most illogical thing in her fearful mind.

She dove under the water to hide herself. Realizing her mistake it was to late. She could see a shadow over the bubbles signaling whoever else was near.

Luckily hse still had her wand so, with great difficulty, she managed a nonverbal bubble head charm so she could still breath.

It solved one of her problems, but she was still trapped naked in the bath with a stranger.

To make matter worse she saw a pair of legs joining the water and then a fully submerged naked lower body.

She had to get out of the situation. She would jump up and hopefully scare whoever it was out.