A person while drinking water or juice says jokingly that he is drinking wine.

Answer

Yes, he has sinned. If someone while drinking water says that he is drinking wine, then he is committing a grievous sin, and it will be recorded that he is drinking wine although it is water or juice. It is haraam to make jokes with haraam and halaal matters. The Deen is not for joking and trifling. Joking with Deeni issues is a kabeerah sin.