The Dungeon's Demon King is the Weakest!?

A boy entrusted with a dungeon by God as the Demon King. He receives various blessings from God, and should be starting his Demon King life!!...However, due to God's careless mistake, he becomes the weakest Demon King in the world. To survive, the Demon King builds his dungeon today as well. This is a story about a somewhat airheaded protagonist who, while taking baths, unfolds adventures with a lonely oni child, an Undine who lives in a bathtub instead of a lake, and princesses who fell in love with the Demon King despite being human.