Recent Studies and Advice
A 2016 study suggests that drinking more water can reduce intake of calories, sugar, salt, saturated fat and cholesterol. Another 2016 study suggests that water instead of diet beverages may help weight loss for diabetics.
A 2015 study concluded that a range of sports, to avoid fatal drops in low blood sodium and hyponatremia (low blood sodium) one should drink when thirsty. NHS Choices advise that as a guide, "... passing pale or clear-coloured urine (wee) is a good sign that you're well hydrated."
The NHS Eatwell Guide says, "drink six to eight glasses of fluid a day. Water, lower fat milk and sugar-free drinks including tea and coffee all count." whereas the National Academies of Sciences, Engineering, and Medicine advises women to have around 2.7 litres daily from all beverages and foods and for men 3.7 litres. One GP, writing in the British Medical Journal in 2011, asserted that there is no evidence to support the NHS advice.
People on medication need to check whether dehydration is one of the side-effects and one should consult a GP to seek advice.
The Advice of the Scholars
As-Suyuti in "Medicine of the Prophet" said, "...the Prophet, may Allah bless him and grant him peace, said, 'The best drink, in this world and the next, is water.'"
Al-Jawziyya in "Medicine of the Prophet" advised that cold water is the most beneficial and to avoid drinking water to break a fast or after sex or a bath or eating fruits or after waking up. He also advised against drinking water just after eating except when necessary and to sip a small amount slowly.
As-Suyuti advises against drinking on a full or empty stomach except when necessary.
Chishti in "The Traditional Healers Handbook" recommends not drinking whilst eating or even a little after, as this disturbs the digestive process, and to have some tea (preferably one that aids digestion, like peppermint) and oranges only after about 30 minutes.
Shaykh Othman Nazih Al Maghribi of the Prophetic Medicine Institute, Canada, reminds us of the Sunnah of the Prophet, peace be upon him, that he used to drink a glass of water with honey stirred in slowly, anti-clockwise, after Fajr Prayers.