Dehydration certainly plays a part in a hangover and thus rehydration can help but a sufficient quantity of alcohol can normally see that idea off.

Funny thing is in the three years that we ran pubs I never once had a hangover. Mrs S kept score on my drinking for a few days and worked out that I was starting at 8am when I sorted the cellar, tasted (tested) barrels of real ale etc. Then I would clean the pipes and, of course, have to taste each one in turn to ensure all was well. Then I would man the bar for the day, breaks for food not required as I also did the cooking for the most part and thus grazed in the kitchen. We never did evening meals so from 5pm to closing I was behind the bar. To avoid offending anyone who said "Have one on me" I always kept "My Glass" under the bar and would simply "top it" at the pump but more often than not the "top" was a pint.
In those days I drank mainly Export Heineken or, if it was a good 'un, the guest ale. Anywhoo madam's calculations showed (as did my waistline) that I was drinking an average of twenty pints a day .... so I swapped "My Glass" for a half pint one and cut back a bit. However two heart attacks later I got the message and we sold up. Now I hardly ever drink.