For a long time the Earth looked like this:
Image: Tim Bertelink CC BY-SA 4.0
There are a few theories about where all the water on our planet came from. One is that during the time depicted above, known as the Hadean, the water was already there. It would have been entirely in the form of water vapor because it was extremely hot but it was around. As the Earth cooled it would have condensed into a liquid, formed clouds, precipitated out of the atmosphere and started to do all the great things we can rely on water today for.
Another theory is that when the Earth formed there was little to no water and we got it from some external source. It might have come from a comet or an asteroid or a series of smaller asteroids. This probably all happened pretty early on in the Earth's history. We can find clues to where the water on Earth came from by looking at the percentages of the isotopes of hydrogen in the water molecules. An isotope is a chemical element that has a different number of neutrons in its nucleus and in this case the stable isotopes of hydrogen can have either zero or one neutron. Scientists today are still collecting samples, analyzing the data and running statistics to try to understand where all the water on our planet came from.
Regardless of where it came from eventually Earth did get water on it, most of it in our one large ocean covering about seventy percent of the surface. Side note: I did say "one ocean." This may be splitting hairs but while most of us are taught that the Earth has a few separate oceans there is only one big body of water that covers the planet. Check out this ArcGIS render of a map by the artist Athelstan Spilhaus:
Everything is connected and you could, in theory, take a boat to any point in the ocean without having to hop out onto dry land.
Anyway, one of the next major events in the history of Earth's water is how it got salty. It did not actually come that way! This bit is fairly straightforward: as the water cycle started to get revved up and rain became a thing we got lakes, ponds and also rivers. All those rivers meant a lot of runoff into the ocean. As water flowed down river it brought sediment with it and in that sediment were a bunch of different salts. Over the millions (and millions) of years that this process continued we eventually ended up with the salinity we currently see in our ocean. Thanks runoff, now I can do a full body float when I go to the beach!
While salt water plays an absolutely enormous role on the Earth's biosphere and climate, its the fresh stuff that we humans and other terrestrial animals need to survive and unfortunately it's much more limited.
You can click to embiggen: the big blue blob represents ocean water, the little blob to its right is all the liquid fresh water and the every tinier blob next to that (which is...really hard to see) is fresh water in lakes and rivers. This water, known as "surface water," is all that is available to us to use as drinking water and water to cook and clean with. It's not very much!
So it's important for us to get together and conserve and protect our fresh water resources. One fun thing you can do is check out your local watershed conservancy, association, council etc. Pretty much wherever you live there is a river near you. A lot of these organizations host clean ups and fun events like festivals and bike rides to get people out there appreciating and connecting with their local waterways. While a lot of those kind of events may be on hold right now, you can still get out there and appreciate the nearest river, lake, pond or stream.
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