I've read several articles over the past year or so that state that coffee is not the diuretic it has been thought to be. What I get from the article that murphmitch posted is that coffee is as much of a diurectic as water--i.e., both will make you pee more if you drink enough of them. If you drink a lot of any beverage, you're going to go to the bathroom more. Plus, it is now thought that coffee may have a number of health benefits, including possibly the prevention of diabetes, heart disease, and Parkinson's Syndrome.
My most recent issue of Nutrition Action newsletter including an interview with a doctor that has authored several textbooks on kidney function and water balance (unfortunately the article isn't available online, but it's in the June 2008 newsletter). He agreed that coffee is not a diuretic in the amounts that people normally drink (same thing for alcohol, incidentally, if you limit yourself to a cocktail or two). He also agreed with Operator265's trainer that the advice to drink two liters of water a day is a myth that has been exploited by the bottled water industry. His advice was to drink what you normally drink with meals and drink in between meals if you're thirsty.
And not only do just about all fluids, flavored or not, count towards your water intake, but so do most foods. Almost everything we eat has some amount of water in it. Even bread has water in it and that water counts towards your daily fluid consumption. One article that has info on this is "
Fluid Fundamentals," which appeared in a recent issue of Cooking Light.
I do think that beverages can be helpful in terms of making you feel full and I generally try to drink an 8 to 12-oz beverage with most meals and snacks, for this reason alone. But I definitely still feel like I'm properly hydrated if I don't do this.