Question

how is the volume of water affected when it boils at 100 °C

Answer 1

The volume of water constantly reduces as it evaporates

Answer 2

If we are boiling water in a container at 100 deg C, the water evaporates from the container into the air as vapor.  Hence the volume of water decreases uniformly and gradually in the container.  After sometime all the water evaporates into the air.

The volume of water increases in the vapor phase. But that is not visible to us. Water expands a lot when it becomes vapour.