If someone feels thirsty they're already dehydrated. The elderly have reduced thirst sensation so are especially prone to dehydration. Tea and coffee (as well as some fizzy drinks) contain caffeine which is a diuretic, so affects the physiology of water reabsorption in the kidneys so they put out more water in the urine contributing to dehydration. Tea can contain just as much or more caffeine than coffee.Water doesn't do that. Dehydration can lead to constipation which isn't good for the bowel and can contribute to irritable bowel, diverticular disease and bowel cancer. Not drinking enough water isn't good for the kidneys and in severe cases can lead to acute kidney failure. A lot of people, especially the elderly, are chronically dehydrated and they're lots of contributing factors such as access to drinks (mobility problems, whilst in hospital! or care homes) ,reluctance to drink due to chronic pain or general lethargy because when they drink adequately they have to walk to the loo, or ask somebody to help them to the loo. Dehydration contributes to confusion,mental "fog", sleepiness (which further contributes to reduced drinking) postural drops in BP and falls,especially in the elderly. Somebody mentioned the colour of the urine as a guide to how well hydrated someone is. Other easy tests are whether the tongue looks nice and pink and moist and if you gently pinch the skin on the back of the hand ,when you leave go the skin shouldn't stay raised up but should return to normal immediately. Rather than the fixed amount you can also use the 30mls of water per kg of body weight per day. Not sure if this link will be allowed but it might be useful regarding nutrition and hydration. www.ageuk.org.uk/brandpartnerglobal/westcumbriavpp/documents/target%20wellbeing/nutrition,%20malnutrition%20and%20hydration%20induction.pdf