His Majesty the Demon King's Housekeeper Vol. 8 (manga)

Sakura’s newest adventure has her heading to the underwater Mermaid’s Domain, where she must find King Emelda a pearl with anti-aging properties. But the Mermaid Queen becomes smitten with Sakura, and the housekeeping saint finds herself trapped underwater with a fishtail! It’s up to the Demon King to dive to her rescue. Will this deep water adventure finally make Sakura realise her deep seated feelings?