Coconut water is the best way to stay hydrated in Mumbai. Coconut vendors are everywhere. All you have to make sure is that you have a clean straw to drink the water.
A young coconut is a coconut that hasn't reached maturity. It has different properties than a mature coconut. The meat in a young coconut is softer and more like gelatin than a mature coconut. For that reason it is sometimes called a tender coconut. There is approximately 1-2 cups of coconut water (not to be confused with coconut milk) in each young coconut.
Coconut water is the liquid endosperm inside young coconuts. As the coconut matures this liquid largely becomes absorbed into the flesh found in mature coconuts. Coconut water has long been a popular drink in the tropics. It is naturally fat-free and low in food energy (16.7 calories or 70 kJ per 100 g). Due to its sterility, pH, mineral, and sugar content, coconut water had been successfully used as liquid in intravenous therapy in emergency situations.