There is not an “immense amount of water” on Earth. To we specks walking like ants on the surface of the planet it looks to be a lot of water, but it really isn’t.
To illustrate this I like to show the image below which shows the total water of all the oceans, lakes and rivers on the planet compared to the planet itself.
You can see from the above image, the amount of water relative to the size of the planet is indeed very little, almost insignificant. In fact, relative to the size of the earth, the ocean is very shallow.
This means that only a very small percentage of the collisions early Earth experienced needed to bring water to the planet. The vast majority of these early collisions did not result in the presence of water.