If the oceans and seas cover two-third (2/3) of the total surface of the Earth, why is there always a shortage of water in the world?can't we purify that water and use for nomal purposes?

 See, the ocean water, is too saline for our use. If we consume them, we will caught deadly diseases, like diohrea, etc. We can only drink and purify freshwater (present in rivers,lakes,etc.) , which cover less than 0.01% of surface area on the earth. We get this freshwater through the evaporation of saline sea water, then condensation, and at last, precipitaion. You might be knowing the water cycle. We can't do it by ourselves. Today, we pollute these sorces of freshwater too much. As a result, their is a shortage of water on the earth.