Why water is precious? And if water is precious then why can't we take the water from sea, ocean ,lakes ,river & underwater ?

It takes a lot of time and money to convert seawater to freshwater. Moreover, If we use water efficiently, most of the water won't turn to dirty, and the measures to convert uncleaned water to freshwater would be greatly reduced. Because of global warming, most of the water gets evaporated and that water inturn joining seawater in the form of rain. Thus we need to save water for future use.