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

Set in the underwater kingdom of Kai'en, which worships the guardian deity of the water dragon, this land is thrown into crisis by mysterious pollution and a plot to forcibly awaken the sleeping water dragon. Rimuru and his crew, originally on a tropical island vacation, are dragged into this deep-sea emergency. Forced to trade their lazy holiday for action, they dive into the azure depths to uncover the conspiracy and prevent the collapse of an underwater civilization.