The water needs of a healthy adult is estimated around 1.5 liters a day.
Our body eliminates around 2.5 liters a day. Food contains about one liter of water that is why we should drink at least 1.5 liters of water to avoid dehydration.
Such quantity of water should be drunk throughout the day without waiting to feel thirsty.
Use cold water from the tap both for drinking and cooking. Hot water can deteriorate its quality as the heat favours bacteria growth.