There are several theories about the origin of water on Earth, but the most widely accepted one is that it came from comets and asteroids. These objects are made of ice and rock, and as they collided with Earth, they delivered water to the planet. This process is thought to have occurred over billions of years, and it gradually filled up the oceans.

Another theory is that water was present on Earth since its formation. As the planet was cooling down, water vapor condensed and formed oceans. However, this theory is less supported by evidence than the comet and asteroid theory.

Finally, some scientists believe that water could have been brought to Earth by volcanic eruptions. However, this theory is also not widely supported, as volcanoes produce only a small amount of water compared to comets and asteroids.

In conclusion, the most likely explanation for the origin of water on Earth is that it came from comets and asteroids. These objects delivered water to the planet over billions of years, and the water gradually filled up the oceans.