ウタカタノミナト

Minato Wadatsumi is a seventeen-year-old high school student raised in a port town by his fisherman father, yet he suffers from an intense phobia of fish stemming from childhood trauma and cannot swim. On his way to school one day, he reluctantly helps a stranded fish by returning it to the sea despite his overwhelming fear. He is suddenly dragged underwater and awakens in the deep-sea world of Latimeria at a depth of 3000 meters, where he encounters Claire, the calm queen who summoned him there because of his act of kindness toward the fish. Claire designates him as a prince and assigns Akari Irie, a pure-hearted and clumsy mermaid with long red hair and a glowing accessory, as his caretaker to help him adjust to life in the underwater realm. As Akari takes Minato to her favorite deeper spot filled with sparkling plankton and minerals that mimic a starry sky, she shares her dream of seeing the real stars on the surface, causing his heart to stir with emerging feelings for her. (Unreviewed summary by Grok 4.1)