Where does Earth's water came from?

The water came from the venture of two gas elements. These are the Hydrogen and Oxygen. If 2 atoms of Hydrogen merge  to 1 atom of Oxygen, then the liquid that will be formed is called water.

Water from Earth cycles from the oceans, where it is evaporated to form clouds then fall from the clouds to land as rain.

Earth’s water originally came from ancient asteroids and comets that collided with our planet billions of years ago, plus water released from molten rock (volcanoes) as steam. Today, water moves endlessly through the water cycle.

The big question here is if the water is infinite.

The origin of Earth's water is about 4.5 billion years ago: Earth formed hot and dry. As it cooled, water-rich asteroids (carbonaceous chondrites) and some comets struck the planet, delivering ice and water vapor. 

Volcanic outgassing: Water trapped inside Earth’s magma escaped as steam during eruptions, cooled, and fell as rain—filling oceans, rivers, and lakes over millions of years.

Earth’s water is mostly as old as the planet itself; very little new water is created today.

Now we have water cycle instead of it origin. The water cycle is the 

Evaporation: Sun turns ocean/lake/river water into vapor that rises.

Condensation: Vapor cools in the sky → forms clouds.

Precipitation: Clouds release water as rain, snow, hail, or sleet.

Collection: Water flows back into oceans, lakes, rivers, or soaks underground as groundwater (aquifers). Earth's water is 97% saltwater (oceans), ~2–3%  freshwater (ice caps/glaciers, groundwater, lakes/rivers), and Rain, tap water, and well water all trace back to precipitation from the water cycle.

(Tagalog)

Saan nanggagaling ang tubig?
Ang tubig ay galing sa pagsasanib ng dalawang elementong gas. Ang mga ito ay ang Hydrogen at Oxygen. Kapag ang dalawang atomo ng Hydrogen ay sumangkap sa isang atomo ng Oxygen, ang lalabas na likido ay tubig.