You don't need to drink so much water daily. 2 litres a day is sufficient. To preserve the moisture content of the skin you need to use a moisturising lotion. In case of very dry skin like yours you will need to use some advanced therapeutic moisturisers. You can get them prescribed by a dermatologist near you. As for dark patches, dry skin can sometimes cause an eczema and lead to dark patches when the eczema is subsiding. They can be treated by anti inflammatory creams or depigmenting creams. Warm regards. Dr. Rajdeep Mysore. Chirag Hospital Bangalore