Although water covers around 70-75% of the Earth's surface, water is actually a rare substance that represents just 0.02 - 0.05 % of the Earth's total mass.the earth has 75% of water so where did that come from then .when the earth cooled down . think about it the earth is in a bubble . H2O did not appear over night .
H2O did not appear overnight as you correctly stated, it is believed that much of the surface water could have appeared as long ago as 4billion years.