The Little Seamstress tells the narrator that the books Luo read to her always made her want to dive. It was a gut reaction. She describes the depth of the pool, and says that Luo's keys always landed in roughly the same spot. She still had to be careful of the stones on the bottom, as some were jagged and sharp.
Remember that diving is something the Little Seamstress has learned how to do since beginning her relationship with Luo. She ftames her acquisition of this skill as a testament to the transformative power of literature.
The Little Seamstress explains that she enjoyed diving to get Luo's keys because she likes pleasing Luo. She says she's not a silly dog playing fetch, or a French girl from Balzac's novels; she just likes diving and making Luo happy.
The Little Seamstress insists that diving is a simple pleasure, not anything that holds deeper meaning. Her desire to please Luo, though, is also indicative of Luo's own self-centeredness.
At the narrator's prodding, the Little Seamstress relates what happened on the last day she and Luo went to the pool. They swam a little and picnicked on the rocks while Luo told her about the heartbreaking part of The Count of Monte Cristo in which the protagonist's lover pretends not to recognize him. Luo and the Little Seamstress decided to act out the characters' reunion. She had a great time, and it was the first time she realized she could play the role of someone else but still be herself. Luo told her she'd make a great actress.
Remember how repressed the Little Seamstress's life has been up to the point of meeting Luo and the narrator. Her revelation here shows that even playing pretend isn't something that occurred to her to do. Her delight in it shows again that such things are universal, and further that they lead to a greater sense of self and maturity.
Then, it was time for the key ring game. Luo threw in his keys and the Little Seamstress dove after them and felt around the bottom of the pool. She touched a snake and retreated to the surface. A few minutes later, she dove back down because she couldn't stand the thought of leaving Luo's keys to a snake. She came back to the surface again, empty handed, before diving down once more. She finally spotted the key ring, but when she reached for it the snake bit her hand. She shows the mark to the narrator, and says the scar will be there the rest of her life.
The Little Seamstress has never expressed fear of the government or its policies; her fears are tangible things like snakes. This is again indicative of her remote and rural upbringing. The snakebite and resulting scar will exist on her body as a physical marker of her coming of age, learning to swim, and her romance with Luo.