"Aqua" is a manga series set in a future where Mars has been terraformed and renamed Aqua, with much of its surface covered in water. The story centers on Akari Mizunashi, a young girl who moves from Manhome (Earth) to the town of Neo-Venezia on Aqua, a city modeled after Venice, Italy. Neo-Venezia is known for its canals and gondolas, and Akari aspires to become a gondolier, known as an Undine, who guides tourists through the city's waterways.

Akari joins the Aria Company, one of the three premier gondola companies in Neo-Venezia, and begins her training under the guidance of Alicia Florence, a skilled and kind Undine. The narrative explores Akari's daily life as she learns the ropes of her new profession, forms friendships, and experiences the beauty and charm of Neo-Venezia. Her interactions with other characters, such as her fellow trainees Aika S. Granzchesta from the Himeya Company and Alice Carroll from the Orange Planet, add depth to the story as they navigate their shared aspirations and challenges.

The series is episodic in nature, with each chapter often focusing on a specific event, encounter, or aspect of life in Neo-Venezia. Themes of friendship, personal growth, and the appreciation of simple joys are woven throughout the narrative. The setting of Aqua, with its serene waterways and nostalgic atmosphere, plays a significant role in shaping the tone of the story, emphasizing a slower, more reflective pace.

Supernatural elements are subtly integrated into the world, such as the presence of Cait Sith, a mysterious cat-like being revered by the people of Aqua, and the occasional occurrence of fantastical events that blend seamlessly with the everyday lives of the characters. These elements add a layer of wonder to the otherwise grounded and slice-of-life narrative.

The characters are central to the story, with Akari's optimistic and curious nature driving much of the plot. Alicia serves as a mentor figure, embodying grace and wisdom, while Aika and Alice provide contrasting personalities that highlight different facets of the Undine experience. The relationships between the characters are portrayed with warmth and authenticity, contributing to the series' overall sense of camaraderie and community.

"Aqua" is a prequel to the series "Aria," and while it shares the same setting and characters, it focuses on Akari's early days in Neo-Venezia, laying the foundation for her journey as an Undine. The series captures the essence of a tranquil, water-filled world and the simple yet profound moments that define life within it.