Water is not a mixture. Water is a pure substance, which is a compound with a constant composition and a fixed ratio of elements. Water is a constant composition of only water molecules with fixed ratios of the atoms of hydrogen and oxygen.
Mixtures contain two or more substances that are not combined chemically and do not exist in fixed ratios to one another. An example of a mixture that may involve water is water and sand. The two substances have different physical properties and can be separated from each other through various methods of filtering. The physical properties of a mixture may change depending on the components, but a pure substance such as water always exhibits the same physical properties.