Question

what is the actual volume occupied by water molecules in 20cm³ of water ​

Answer 1

Taking a different approach to a question that apparently answers itself, are you asking what volume would be occupied by the water molecules, ignoring the space between them, when in liquid form?

In this case it’s not really possible to answer. If you freeze the water to a solid, there is actually more space between the water molecules than at 4 degrees Celsius, the practical outcome of which is that ice floats.

You have to regard the space between the molecules as part of the water for all practical purposes. Ignoring this, you get to the atomic level and atoms are mostly space, even if you could pin down exactly where and electron is, which you can’t.