The mass of Earth’s oceans is about 1.4 x 1021 kg. (How do we know this? See Activity 1, problem 1). But there appears to be far more water in Earth’s interior, something between 10 and 50 oceans worth. It is likely that most of this water accreted within the Earth early in its history and that, in steady-state, some 5 to 10% remains on the surface in the ocean-atmosphere system. Although some evidence supports water delivery by later cometary or meteoritic (asteroidal) impacts, it is likely that surface water was accreted early and outgassed from within the Earth. Nonetheless, some new observations of comets (comet LINEAR) provide new support for cometary origins of water on planets in the inner solar system.
Want to learn more?
See "A Taste for Comet Water."
Question 3: Why are there oceans on Earth and not now on Mars or Venus?
Those two planets also likely accreted much water during their formation as well as having been bombarded by comets just as the Earth was.
Reading assignment(this is the same paper you read for background on the Habitable Zone)
Question 4: Why is liquid water on the surface of a planet important?
In a poll of the readers of Astrobiology Magazine, a scientific journal, 41% rated liquid water as the key factor needed to make a planet habitable, followed by a combination of all other candidate elements [nutrient, water, oxygen, ozone, photosynthetic sources like sunlight, and carbon dioxide].