In a recent Gallup poll, six out of 10 people said they drank fewer than four glasses of water a day, less than half the recommended amount of one and a half litres. One in 25 young people aged 16-24 showed signs of dehydration on the "pinch" test - checking the elasticity of the skin on the back of the hand.
The poll is admittedly suspect as it was conducted for the mineral water industry, which has an obvious interest in these matters. But there is plenty of evidence for the importance of water in preventing constipation, headaches, lethargy - and thirst. Dehydration is debilitating and water may be one of the most effective defences against disease.
The average person loses 1.6 litres of fluids on a cool day while at rest - around a third through the skin and just under a third through the lungs. Most of the rest is lost in the urine though this is more dependent on how much is drunk.
On a hot day, or under exertion, the loss is greater. Moderate dehydration - defined as a loss of 2 per cent of body weight - can occur easily. If the fluid lost is not replenished it reduces filtering in the kidneys and the waste products contained in urine and in sweat are more concentrated.
Dr Susan Shirreffs, of the Biomedical Sciences Department at the University of Aberdeen, who studied the effects of dehydration in the laboratory, said those affected are often unaware of the problem. "Even low levels of dehydration can cause headaches, lethargy, lack of alertness and mood changes. The longer term implications can be more severe with changes in the renal system and cardiovascular system as well as in mental functioning."
Chronic dehydration is thought to have a role in asthma, high blood pressure, indigestion, backache and pains in the joints. But it is not good enough to rely on thirst to tell you when to drink. The sense of thirst declines with age, and the elderly are at greatest risk of dehydration.
For the working population a key problem is that the offices tend to be dry. Most people think tea or coffee is as good for rehydration as water. But drinks containing caffeine act as a diuretic, stimulating urination and increasing loss of fluid. And alcohol is a stronger diuretic than caffeine.
Water is important for the skin - nature's best moisturiser. It also aids digestion and prevents constipation.
A pint or two of water is also the best cure for a hangover. It is key to treating infections of all sorts - flushing out the toxins. Now there is hard scientific evidence it protects against the fourth most common cancer - of the bladder.
Six cups of water a day halved the rate of bladder cancer in men compared with those who drank less than one cup, according to a major study published in the New England Journal of Medicine.
The 10-year investigation of 48,000 men found it was not only water that was protective. Those who drank more than two and a half litres of fluid a day - including coffee, tea and alcohol - also had half the risk of developing the cancer. With every extra drink, their risk fell by 7 per cent.
The Journal compared the finding to the discovery that cigarettes caused lung cancer and the sun caused skin cancer. "Now comes a seemingly simple way to reduce the risk of bladder cancer, the fourth most common in the US: drink more fluids," it said.
There are more than 9,000 new cases of bladder cancer and 3,500 deaths in the UK a year. So if people could be persuaded to drink more, we might save upwards of 1,500 lives.
The protective effect of water is simple to explain. The authors of the study, Dr Dominique Michaud and colleagues at the Harvard School of Public Health, Boston, US, suggest that drinking plenty of fluids dilutes cancer- causing chemicals in the urine.
"Concentrated urine, or less frequent micturition [peeing], will increase the exposure of the bladder urothelium [lining] to urinary carcinogens."
The mineral water industry claims that as a nation, Britain has much to learn from the rest of Europe since we drink nine times less bottled water than the Italians. But there is no need to drink bottled water to top up your fluid level. Tap water will do just as well.
Publishing the Drinking Water Inspectorate's ninth annual report, which showed that 99.78 per cent of samples of tap water tested complied with national standards, Michael Rouse, chief inspector of the nation's tap water said the overall standards of drinking water in England and Wales are now so high that if customers were to stand a jug of tap water in the fridge for a couple of hours to let any chlorine clear, "they will never be able to taste the difference between that and the water they buy in bottles."