A pack of raisins.
Yes, I've read your question through and through. Please, bear with me for a while, to see the longer answer:
The problem with concentration when you haven't eaten arises primarily because of the lack of nutrients, i.e. glucose.
Glucose is virtually the sole fuel for the human brain, except during prolonged starvation. The brain lacks fuel stores and hence requires a continuous supply of glucose. [...] Fatty acids do not serve as fuel for the brain, because they are bound to albumin in plasma and so do not traverse the blood-brain barrier. In starvation, ketone bodies generated by the liver partly replace glucose as fuel for the brain. (1)
So basically, you can't focus because your brain lacks fuel. It's somewhat similar to a car: yes, you can add water to the radiator (and need to), and yes you can add sparks to make your engine start, but without fuel it just won't run.
Caffeine will not only fail to solve your problem, but can actually be harmful on an empty stomach. It can hurt both your stomach and can make you feel even less focused after a short while.
Caffeine increases energy metabolism throughout the brain but decreases at the same time cerebral blood flow, inducing a relative brain hypoperfusion. (2).
Caffeine may increase an individual's sensitivity to hypoglycemia through the combined effects of reducing substrate delivery to the brain via constriction of the cerebral arteries, whilst simultaneously increasing brain glucose metabolism and augmenting catecholamine production. [...] Under laboratory conditions, acute ingestion of caffeine markedly enhances the symptomatic and sympathoadrenal responses to hypoglycemia in both healthy volunteers and patients with type 1 diabetes. (3)
To sum it up - caffeine will make you more alert for a short while, but it will eat up little fuel that you had left, and leave you almost completely without it. For the half hour, 40 minutes perhaps, you will feel better, but then the problems will start - even stronger sense of hunger, worse problems with focusing on your work and possibly light-headedness and hand trembling might occur.
Heartburn is the most frequently reported symptom after coffee drinking. It is demonstrated that coffee promotes gastro-oesophageal reflux. Coffee stimulates gastrin release and gastric acid secretion [...] Coffee induces cholecystokinin release and gallbladder contraction[...] (4)
Caffeine will stimulate the release of gastric acid and bile, which are meant to digest food. But, since there will be no food to digest it will begin to "digest" your stomach instead (well, not literally, but it can cause you pain and problems, especially if you make a regular practice of it).
Water might leave you feeling fuller for a while, but it won't solve the underlying problem (lack of brain fuel), and large quantities of water, drank in a relatively short time span, on an empty stomach, might leave some feeling a bit sick (the smaller the stomach volume, the worse it gets). It's not dangerous, but can be uncomfortable.
- Do you ever forget your keys? Your mobile phone? Well make a habit of not forgetting your snacks. It is simply a matter of priorities, and health is a priority. (And do make sure that the snacks are as healthy as possible).
- Have a contingency snack plan: keep a bag of raisins (e.g.) in your purse. If you change purses, keep a bag of raisins in every purse, or next to something that you won't forget (e.g. your keys).
- Keep a pack of raisins (or something similar) at work (in your desk perhaps).
For 2 and 3 - just remember to occasionally check those places and expire dates - no one likes to find a 10-month old bag of raisins in their purse/desk/key drawer.
Each Organ Has a Unique Metabolic Profile
Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects.
The best defense against hypoglycemia is to recognize it: is caffeine useful?
Coffee and gastrointestinal function: facts and fiction. A review.
** an important note: ref. 3 discusses caffeine as an indicator of hypoglycaemia in diabetic patients, BUT I can access and link just the abstract and there is no room in it to discuss how dangerous hypoglycaemia is in patients who receive insulin therapy - in these cases hypoglycaemia can be deadly, and something that would amplify the symptoms is used only so they can recognize the condition and eat something. Without food to follow immediately, caffeine is not a good solution.