Water is the most precious resource on our planet, a key ingredient together with air for development of all life on Earth, in fact even our bodies are mostly water so when you think about it you have to ask yourself a question what is the origin of all this immense amount water in our planet, and how were our oceans formed.
If we go way back in the past, around 4.5 billion years ago, we could see the Earth was bequeathed with sufficient water for oceans to form and for life to find favorable areas in the seas and on the continents that resulted from plate tectonics. But how did water arrive on our planet and filled this gigantic oceans?
The most accepted theory suggests that oceans were formed from vapors emitted during intense volcanism that happened at the time when our planet appeared in endless space. According to this theory both the ocean as well as the atmosphere were formed from volcanic gases that originated in Earth's interior. According to this theory water was key part of Earth's initial inventory, but not all scientists believe in this theory. There are some scientists like Francis Albarède of the Laboratoire des Sciences de la Terre that believe how water in fact came much later on our planet from the turbulence caused in the outer Solar System by giant planets. According to his theory the ice-covered asteroids thus reached the Earth one hundred million years after the birth of the planets.
This would of course mean that the origin of water on our planet is extraterrestrial,and could have arrived on our planet much later than dominant theories believe it had. The main thesis of his theory is that during the formation of the Solar System, the temperature between the Sun and the orbit of Jupiter never dropped enough for volatile elements to be able to condense with planetary material. The arrival of water on Earth therefore has to correspond to a later episode in Earth's history.
The water on our planet could have its origin in a space phenomenon that happened some tens of millions of years after the lunar impact, namely the big clean up of the outer Solar System initiated by many giant planets. These planets had very strong gravity, and so were able to sent the final ice-rich planetary bodies in all directions, including the direction of our planet.
These ice-rich planetary bodies then penetrated into the mantle through the surface and so the plate tectonics began creating continents surrounded by vast oceans. This theory would even explained why there is no life on Mars because Mars probably Mars dried out before water managed to penetrate in depth.
In any case this theory is rather interesting, and what ever caused water to flow on our planet enabled something to our planet that no other planet in Solar system has, it enabled Earth to harbor life.