What problem does Aimee have with her waterproof legs?

Her wooden legs float to the surface of the water.

Explanation:

Aimee Mullin's real-life story "A Work In Progress," tells of how she began to accept her life with 'fake legs' after hers were amputated during her earlier years. The short story tells of her journey to accepting her life with the "legs" made for her.

When she first got her "wooden legs" that wouldn't rust, she was "so excited to get these legs". Then she found put that "these legs were so good at being waterproof that they were buoyant", which led her to admit "They were the bane of my existence". Thus began her trouble with the waterproof legs, which would immediately bring her up the water even after a high dive.