Everyone is different, and their fluid requirement is specific to them. As a fitness instructor we were told, once someone is thirsty, it is too late: you are dehydrated, so small quantities should be taken throughout the workout/day. Not a litre at a time, unless you have a health problem. Hydration is indicated by the colour of your urine: it should be a very pale straw, which it will be whether you drink tea, coffee, milk, juice, soup, salads, whatever.
If you drink fluids with your meal, I understood that it prevents efficient break down of the foods into its component parts. And a friend said that if she drank water, she couldn't eat as much. As she was eighty, and skinny, I felt that she should be drinking her fluids between her meals.
The problem with tap water is that it is chlorinated, and chlorine is toxic: for some people even in such tiny quantities, they could well be sensitive to that amount of chlorine.
Alcohol dehydrates more than caffeine. I used to only drink wine at weekends, and at weekends, guaranteed, I would get leg cramps at night. Cramp is an indication of dehydration. I thought that was pretty telling!
Dehydration becomes more of an issue as we age: we dessicate, which is why we are more likely to get cystitis. If you catch it early enough, and drink enough (boring!) water, you could possibly avoid taking anti-biotics. Another suggestion is to take a teaspoon of sodium bicarbonate (an alkali)in a glass of water 2-3 times day - suggested by my GP. I avoided anti-biotics that time.
And I have been constipated for years, despite drinking at least three litres of liquid a day (4 mugs herb tea, and a minimum of 8 mugs weak tea with milk,) and a minimum of an hour and half hours of exercise per day. Our requirements are so individual.