I don't want to throw cold water on anyone's favorite anecdote, but there has been quite a lot of information out there lately showing that the old "eight glasses of water a day" (roughly two liters) is really in the category of urban myth.
The simple fact is that we get a great deal of the hydration we need from the water in our food. Most people get significantly more than the proverbial eight glasses simply through normal eating.
A 1995 survey in Australia found that on average women ingest about 2.8 liters a day, and men about 3.4 liters.
Even worse, some of the so-called "research" that seems to show the need to drink a lot of water was sponsored by the companies that sell bottled water.
The more reputable research shows that the proper approach is just to take a drink when you start to feel thirsty.
Incidentally, beer is roughly 95% water, so it counts!
Alcohol doesn't dehydrate you any more than coffee or tea.