In order to answer this question you have to consider not only all the water bodies, such as oceans, lakes or rivers (people managed to calculate these areas only in the 20th century). To find the total of the Earth's hydrosphere you also have to know the precise amount of water in ice caps and glaciers.

The approximate area of Earth's water is about 1386 million km2. It's unbelievable, but the fresh water resources are only 2% of the total amount. Nevertheless, even this modest volume is enough for humanity to go on living comfortably.