That you set the water up to use a custom stencil so you can replace colors or isolate the material.
Is it possible to get a location between the two that would allow for a waterline?
Yes and no.
If you build out the function you can call it out and get height at x/y locations.
Sampling what is in front of the camera could become expensive on the cpu (too many pixels at 4096).
If the camera is stuck/unable to tilt you can make a vertical mesh with a fairly heavy amout of tris that runs the math on the top row of vertex.
This however will likely never match your water sheet as the vertex are in a different locations, so while a good idea for a still camera, and it works for a moving one too, it is very likely you notice issues while moving along…
The stencil way has much less artifacts since it directly uses the water sheet.