When considering most of the things that predate man, there is never a concrete fact as to how they happened. There are only speculations and theories that seem mostly possible. As for origin of water on Earth, there are two theories that seem the most possible and they are the extra planetary source and the internal source of water.

The extra planetary source says that water on Earth came from outer space on meteoroids or comets that came from Neptune. Neptune is a planet in our solar system whose surface is covered in an ice and water layer. Because of this, it's possible that the water on this planet came from the water there. There are also water-rich meteoroids that may have landed on earth billions of years ago.

The internal source says that water came from the gradual dehydration of hydrated minerals here on Earth. Or at least a part of it. Then it's also possible that volcanic eruptions forced water vapour in the atmosphere to condense and fall as rain, thus creating the water cycle.

As I said, these are just theories because they cannot be proven but still they are the two most considerable answers to the question