Where did the ocean come from?

     

          Earth first formed about 4.5 billion years ago. At first, it was too hot to have an ocean. Any water would have boiled right off. After Earth cooled down, the water would stop boiling off into space. Any water would actually stick around.
The rocky material that formed Earth in the first place contained some water. But that probably doesn't account for all the water. 
Comets are mostly water ice. It could be that comets made regular water deliveries to Earth.
It would take a lot of comets to fill the ocean! But comets could well have made a big contribution. Asteroids contain some water too, so they may have contributed.