The Earth is a big place and nearly three quarters of it is covered in water - surely that's enough for everyone?
It is now estimated we have over 3,000,000,000,000,000,000 gallons of it, dispersed not only in oceans, rivers and lakes but also under the ground as well as in the air and clouds. This amount of water is kept constant due the continuous water (or hydrologic) cycle of the earth. As rain falls, it fills the lakes and rivers which flow into the sea where it evaporates into the air as water vapour, forms into clouds and deposits it back again as rainfall.
Oceans contain about 98% of the total amount of water on earth, with 1.6% of the remaining 2% being locked up in the polar ice caps and glaciers. About 0.36% of the remaining 0.4% is out of sight locked away in aquifer rocks and reservoirs, and 0.01% is actually in the air and clouds leaving just 0.03% in our lakes, rivers and reservoirs.
Saline water is of course useless for human consumption and agriculture. Of course there are many millions of salt water life forms including mammals and fish, dependent on salt water, so getting the balance right is vital. The reason for the sea's salinity in the first place is that the Earths' rocks contain sodium and chlorine ions which are collected by the acidic rain (rain that has absorbed some carbon from the atmosphere) and then eventually washed into the sea. In addition to the sodium chloride added to the oceans in this way, the sea beds are continually producing salt via underwater eruptions. Luckily for us, evaporated sea water vapour leaves the salt behind thus the sea remains saline, and may actually be getting more salty. The sea does, however, get rid of a lot of its salt due to the millions of sea creatures that absorb it and after they die, there bones and shells finish up on the sea bed as sediment. Eventually this sediment turns into rock and, where it has risen above sea level over millions of years, this sedimentary rock - in particular limestone - is dissolved by acidic rain and finds its way back into the oceans.
There is currently a worldwide problem with fresh water shortages due to general wastage, increased agriculture and the rise in population - It may surprise some that the average human in the western hemisphere uses over 30 gallons of water every day. Desalination plants would appear to offer some answer to water shortages but whilst they produce some 0.3 percent of the planet's fresh water, but at the current time they are very inefficient energy wise.
The big problem for life dependent on both fresh water and salt water is of course pollution, and whilst the Earths' water remains constant, its quality is quite another matter. Pollution of the world's water supply from industrial processes and agriculture is a problem that needs to be resolved soon - and quickly.