So I have gained an additional 10 pounds since I got married. Possibly one of the by-products of newly weds out of the single scene.
So I have decided to go on a diet. My wife told me I have to drink lots of water before I eat and after even a coffee. I asked about the coffee and she said because it dehydrates.
So I decided to look that up as I have heard it before.
THE CLAIM: Coffee is a diuretic (meaning, it dehydrates you)
In 1928 a study by Lawrence Armstrong, a professor in the Department of Kinesiology at the University of Connecticut and director of the Human Performance Laboratory was the first look into this claim. He concluded that it had an increase inurination in people who drank caffeinated beverages, and suggested that caffeine was a diuretic.
The notion that you have to drink an extra glass of water for every cup of coffee you drink is one of the most persistent urban legends in nutrition.
The study in my opinion was flawed
— suggesting that giving people a single dose of caffeine (typically as a pill) can trigger a small increase in urine output (read: they pee more). That might make it sound like caffeine does, in fact, cause dehydration, but there is more to the story.
If chronic coffee consumption really did cause us to become dehydrated, then a lot of people would end up needing fluids administered by a hospital.
First, the studies that demonstrated some type of dehydration-like effect typically used relatively large doses of caffeine (500 mg or more), or about the equivalent of drinking 3.5 cups of coffee, or 10 cups of tea, all at once.
Second, any diuretic effect seems to be most pronounced in those who don’t consume coffee regularly. Intuitively, this makes sense: After all, if chronic coffee consumption really did cause us to become dehydrated, then an awful lot of people would be end up hospitalized, or dead, due to dehydration every day.
There is a third important consideration with caffeine research, and that is that relatively few studies use real coffee; most use caffeine, which is just one component of coffee. We know that coffee is a fantastically complex beverage (it contains dietary minerals such as magnesium, as well as numerous antioxidants and numerous active compounds, all of which can have an effect on the body. And its properties change quite significantly with the way it is prepared — for example, whether it is filtered or unfiltered).
If you only drink coffee occasionally, then you might want to add an extra glass of water on the days you do grab a cup.
Published in the journal PLoS ONE, sheds some more light in the subject:
In the study, researchers examined the effect of drinking a moderate amount of coffee on 52 healthy men who were regular coffee drinkers at the start of the study. 
During the study, the men were given 200 mL (roughly 7 ounces) of either plain water or coffee, four times per day, while keeping the rest of their eating and drinking habits constant. After three days of this routine, the researchers found no difference in any measurement related to hydration, including urine output or total body water (considered a gold standard indicator of hydration status).
Conclusion: Mostly Myth