Sigrid loves to explore the rocky coastlines and caves of her hometown, looking for slugs, shellfish, and barnacles. "It's research!" she protests, no matter how many times her parents tell her to stop getting her sweaters and leggings muddy. She is relentless about becoming a marine biologist.
One day she meets Nimue, a water elemental.
...or a selkie, or a rusalka, or a mermaid depending on where you're from. Well, it's folklore so I allowed myself some creative fact-blurring.