https://www.scientificamerican.com/article/how-did-water-get-on-earth/

About 70% of the surface of our planet Earth is covered in water. We are nestled in our solar system at just the right distance from the Sun for this liquid water to exist. Any farther and that water would be frozen in ice. Any closer and temperatures would be too hot and we would be at risk for a runaway greenhouse effect similar to what’s happening on the scorching surface of Venus. Our not-too-cold, not-too-hot position in the so-called ”Goldilocks zone” is a pretty good thing because, of course, water is necessary for life. 

But how did that water get here? Water is a defining characteristic of our planet and it plays such an important part of our daily lives. Understanding how water arrived on Earth is a key part of understanding how and when life evolved here as well. But we don’t even know how it where it came from. Scientists are still actively researching how our planet got to be so wet in the first place.