욕실이 어디예요? (Where is the bathroom?)
욕실 쓰고 싶어요/화장실 써야 해요. (I need to use the bathroom.)
욕실 means bathroom, but Korean houses nornally contain a loo in the bathroom, so you can say 화장실 instead. But when you mean only BATH not LOO, then you have to use 욕실.