Hey, Water You Doing? (S07E08)

Disappointed by the lackluster submarine ride at Sassy Cat amusement park, Billy, Mandy, and Grim embark on a real underwater adventure thanks to Grim's magic. However, they soon find themselves facing execution for invading the Googen Kingdom, ruled by King Triceps, a merman with a wrestling background. The king’s daughter, Hariel, a mermaid, intervenes to save them and proposes marriage to Billy to claim the throne. It turns out that King Triceps is actually Hariel's sister, the cunning sea witch Uvula, leading to a fierce rivalry. The chaos concludes when Billy agrees to marry both sisters. Ultimately, it’s revealed that this tale is just a story Billy is recounting to Miss Butterbean, who inquires about his missing report.