This is because water is a very good solvent, which means that many kinds of materials can dissolve or wash away in it.
Sea water is salty because salts are dissolved in it.
The beverages we drink are water with something dissolved in it. When we wash something, the water dissolves and carries away the dirt. Because water is good at dissolving things, it also helps to transport them in dissolved form from one place to another. These qualities which make water so useful also make it easy for water to be polluted, or made dirty with things that are dangerous or harmful. Often pollution comes from man-made substances or things produced by man s activities that prevent the water from being used naturally.