According to the plot information you provided, I recommend Return of Trash: The Mermaid Queen of the Underworld King to you. This book has a love story between a trash mermaid princess and an ominous son who was chased out of the Dragon Palace. Although it doesn't completely meet the requirements you provided, there is indeed a scene where the protagonist is locked in someone's pool. It may match your memories. I hope you like this fairy's recommendation. Muah ~😗