That Time I Got Reincarnated as a Slime: Tears of the Azure Sea

Set in the underwater kingdom of Kai'en, a realm that worships the guardian water dragon, a mysterious pollution and a plot to forcibly awaken the sleeping dragon plunge the nation into crisis. Rimuru and his crew, originally on a tropical vacation, get dragged into this deep-sea emergency. Forced to trade their lazy holiday for serious business, they dive into the azure depths to uncover the conspiracy and prevent the downfall of an underwater civilization.