If you consume water you can live for a greatly extended period of time. So it definitely made it better. Any fat tissue you may have had stored would probably have been broken down within the first 24 hours of your starvation period (the moment when you have ZERO food in your stomach), after that your body begins to break itself down to keep vital organs functioning. If you consumed water it would slow down the process dramatically. It is however still highly dangerous to starve oneself, since you don't actually die from the lack of food itself (usually), but rather the amalgram of diseases you contract as a result of the lack of food.
*snerk*. It's rare to starve to death--even on a total lack of nutrition--in anything less than a month. Extreme cases can last more than two:
"Generally, it appears as though humans can survive without any food for 30-40 days, as long as they are properly hydrated. Severe symptoms of starvation begin around 35-40 days, and as highlighted by the hunger strikers of the Maze Prison in Belfast in the 1980s, death can occur at around 45 to 61 days."
It is actually possible to starve to death without ever completely using up your adipose tissue. Obese mice do this--they are so hormonally geared to store fat that they will completely cannibalize their organs before decreasing their adipose tissue. No living human being has so little body fat as to be able to consume all of it in a 24 hour period--a pound of fat is worth about 3500 calories, more than you use in a day when resting. Even a VERY lean person (<5% body fat) is probably carrying upwards of 20 lbs. of pure fat on their bodies.
Maintaining muscle tone under those circumstances is pretty incredible, though, particularly for an adult male. Muscle uses up an enormous amount of calories in self-maintenance and requires a pretty much constant input of protein. Your body NEEDS protein and when it's not getting it from food, it starts pulling it out of those metabolically expensive muscles. Adult human males are protein furnaces. If you ever watch a show like Naked and Afraid where the contestants do regularly go up to 3 weeks without food, the men start out strong but after a few days of deprivation they aren't good for much. The women usually feel the poor conditions worst at the beginning and finish out strong--the first solo finisher was a woman. We generally have more body fat and less muscle mass to maintain so we starve better than men do.
So, from that perspective, Sten's ability to go from 3+ weeks of starvation to violent activity was pretty noteworthy. It's possible Qunari have more efficient protein usage than humans do (a good possibility considering their extra size and muscle mass). Or he may be able to enter a hibernation state that slows the metabolism dramatically (a lot of critters can do this). Or he may have a sugar-synthesizing pigment in his skin that, like chlorophyll, can effectively turn sunlight into energy. Or he may just be so overly fit in his natural state that 3+ weeks of starvation still won't reduce him below the human capability threshhold. Or he was exaggerating because he didn't give a crap whether he got killed or not.