Okay, so 'Aquamarine'—there's a couple books with that title. Assuming you mean Alice Hoffman's YA novel? It's basically a mermaid story, but not a dark, fairy-tale one. Two best friends, Hailey and Claire, are facing their last summer together at a rundown beach club. They find a mermaid, Aquamarine, washed up in a pool after a storm. The main plot revolves around them trying to help her get back to the sea, but also trying to get her to help them with something: proving that love exists, because they have a crush on a lifeguard named Raymond. It's a short, bittersweet summer fable about friendship, change, and believing in magic.
It's super short, almost a novella. The real heart isn't the mermaid's quest home—it's the girls dealing with their impending separation. Helping Aquamarine is this last, shared adventure that distracts from the sadness. Hoffman’s writing has that dreamy, lyrical quality, so even the simple plot feels weightless and poignant. The ending is quiet and perfect for what the book is.