How to mask space under the mesh?

Hello everyone, I want to create a waterline effect but am having trouble finding an area that is underwater and which is not.
What's the best way to do this?
I can see the waterline effect is created by post-processing. The only problem is masking the area underwater.
Z-buffer is probably not suitable as some of the water pixels may not be visible on the screen.
I wonder if generating a 3d mesh under the waterline is the right way to do this?
What better way to mask this area?

Related Topics:
https://forum.unity.com/threads/waterline-underwater-splitscreen.1007648/
https://forum.unity.com/threads/abo...o-share-even-a-hint-of-how-to-make-it.944373/