How Earth came to have water:

http://phys.org/news/2014-02-earth.html

Comets and water ice asteroids early on carried water to Earth and even the heat of the magma was not enough to drive it all away. It sounds like there was ten or 20 times more water originally but the magma oceans 4 billion years ago took out most of it but left enough to condense back to liquid when the planet cooled enough to allow liquid water.