When a water volume is scaled on it’s Z axis, there will be a small line of “underwater color” that appears above the water. This line’s position is a direct mirror of the bottom of the water volume’s box. However, this ONLY happens if both you and the volume are in a negative Z coordinate position.
Steps to Reproduce
Go under the map in a condo (or anywhere in a condo with a -Z coordinate.
Place a Water Volume.
Noclip or stand in a spot above the volume.
Slowly scale the Water Volume until your screen goes underwater.