While it is likely that Earth got a good amount of water, and other materials, from comets, the probable incidence of comet collisions with Earth in its distant past, in a period called the ‘Late Heavy Bombardment’, falls well short of explaining the amount of water that we find on our planet.

In fact, Earth’s ocean water is indeed much more similar to water found in asteroids and thus seems to account for one of the more significant origins of water on our planet.

The similarity lies in the ratios of different types of hydrogen that are prevalent in different bodies of water in the solar system.

In this regard, the ratios of protium (the more common and lightest isotope of hydrogen, typically simply referred to as hydrogen for this reason) to deuterium (a heavier isotope and less common, with one proton and one neutron in the hydrogen nucleus) are important because they can be used to work out the likely origin of different bodies of water.

The deuterium to hydrogen ratio for water on Earth is indeed more similar to that typically found in asteroids.