There could be a romantic relationship. After being saved by Hermione, Draco may develop feelings for her. He might be drawn to her kindness and strength. At first, Hermione might be hesitant, given their past differences, but over time, they could fall in love. This could lead to them having to face opposition from their families and friends who still hold grudges from the past.
The plot could start with Hermione being in a dangerous situation near water, like she might be attacked by a water - based dark creature while near a lake at Hogwarts. Draco, who might be nearby for some reason, overcomes his initial hesitations (due to their past enmity) and jumps in to save her. After that, they could start to see each other in a new light, leading to a slow - burn relationship as they deal with the aftermath of the incident and the reactions of their friends and the Hogwarts community.
You could try searching on popular fanfic websites like Fanfiction.net or Archive of Our Own (AO3). Just type in the exact title or relevant keywords like 'Draco saves Hermione drowning' in their search bars, and you might find the fanfic you're looking for.
Another theme is the changing of relationships. Their relationship, which was mostly adversarial in the books, takes a new turn. It can range from friendship to a more romantic relationship. For example, in many fanfics, the act of saving Hermione makes Draco realize his feelings for her that he had been suppressing.
Draco might be motivated by jealousy. He could be jealous of Ron and Hermione's friendship or a possible romantic relationship between them. So he tries to harm Hermione out of spite.
Well, in such a fanfic, it might show how Draco and Hermione's relationship evolves over time. It could start with them being at odds due to their different houses and family backgrounds. But as the story progresses, they might find common ground. For example, they could both be interested in a particular area of magic that forces them to work together, and through that, their feelings start to change.
One popular 'Draco Saves Hermione' fanfic is 'The Green Girl' on FanFiction.net. It has an interesting plot where Draco's actions to save Hermione lead to a series of unexpected events that change their relationship dynamics in the wizarding world.
One common theme is redemption. Draco, who was often seen as a bit of a bully in the original series, gets a chance to redeem himself by saving Hermione. It's about him becoming a better person.
Well, perhaps Snape uses his position as a teacher to step in. He could have been observing the situation secretly and when Draco was about to do something harmful to Hermione, Snape intervened with a firm command or some magic spell to stop Draco in his tracks.