4.26 Ocean - player inside sub / boat

Hoping to use the Ocean body, but need to alter the water line (the visible “edge” of the water) so that a player inside a vessel that submerges will see the water / post process start on the windows, NOT directly in front of the camera.

Tried this:

But not sure I can get it working because of the way the ocean bodies and post process volumes are working.

Also watched this Inside Unreal, where they talk about mesh-based post process?

But am not entirely certain how to set that up.

Help? Thanks!