Hangovers ain't just to do with dehydration and coffee doesn't dehydrate you, at least not straight away, caffeine does but there is a fair amount more than caffeine in coffee, mostly water in fact, which hydrates you. Same goes for alcohol, it too is a diuretic but beer and spirits will still hydrate you with all the water in them, later on you might pee out more than normal though which could lead to dehydration if you're not topping up with water or something with water in it.
Hangovers ain't just to do with dehydration and coffee doesn't dehydrate you, at least not straight away, caffeine does but there is a fair amount more than caffeine in coffee, mostly water in fact, which hydrates you. Same goes for alcohol, it too is a diuretic but beer and spirits will still hydrate you with all the water in them, later on you might pee out more than normal though which could lead to dehydration if you're not topping up with water or something with water in it.