One molecule of water is a compound of two atoms of Hydrogen and one atom of Oxygen. There are three isotopes of Hydrogen found in nature. These are Protium, Deuterium and Tritium. By isotopes we mean the same elements having same atomic number but different atomic weights. Protium is H11, Deuterium is H12 and Tritium is H13. The number noted below represents atomic number of the element and the number noted above represents atomic weight of the element. Protium, Deuterium and Tritium are denoted by the symbol H, D and T respectively. There may also be mixed water containing one atom of Protium and one atom of Detuterium or one atom of Protium and one atom of Tritium in their molecules. Hence, there are waters HDO, HTO and DTO also.
There are again 3 isotopes of Oxygen that exist in nature. They are O816 , O817 and O818. The first being by far the most common. Taking into account these varieties of Oxygen, there are again 12 kinds of water. So totally there are 18 kinds of water. The lightest water is H2O16 and the heaviest water is T2O18. So, whenever we drink a glass of water, there may not be one kind of water in our glass.
These waters differ perceptibly in their properties. They have different diversities, different freezing and different boiling points. Different kind of water in nature have different relative contents of these water. D2O is generally called Heavy water. Tap water contains 150 gram of D2O per tonne. But the water of Pacific Ocean contains 165 gram per tonne. Isotopic composition of water is different every where. Always there is isotopic exchange occurring in nature continuously. Different isotopes of hydrogen and Oxygen keep replacing one another under different conditions. Mainly we deal with protium water. But D2O is also used in Nuclear Reactors for moderating Neutrons. Scientists use various kinds of waters for investigation purposes.
Besides Natural occurring isotopes of Hydrogen and Oxygen there are also man made different radioactive isotopes of Oxygen likely O14, O15, O19 and O20. And similarly there are man made radio active isotopes of Hydrogen likely H 4 and H 5. Taking into account these man made isotopes of Hydrogen and Oxygen the list of different kinds of waters increases to over 100. Thus, we can imagine the different kinds of water. In stead of saying a glass of water, we may say a glass of waters.