Richard tried to thrash and kick her off, but the most he ended up doing was cause them to sink further into the water. By the time he stopped moving, the crest of the waves they had started when they ended up in the water would run over her shoulders and brush her chin. To be sure he was dead, Hermione pushed the rest of his body under the water and when she didn't see his body rise, indicating that his lungs were no longer full of air, but water, she allowed herself to smile.