Parallax refraction problem

I’m interested in recreating this effect:

FYI the video description states “I’m using parallax mapping to fake the ground underneath the water and displacement for the waves. The caustics/refraction is created by using the wave height to warp the UVs of the sand texture underneath.”. I’m unsure how to tie the displacement map to the UVs.

To be clear, this is the sort of refractive disturbance I wish to create:


My intention is ultimately to have a gradient “underneath” tiled with a displacement map panning together (with some sort of more erratic movement on top). One of the other problems I had with using bump offset to attempt fake refraction was having the “underneath” texture visibly tiling as it was being rendered so far below the surface, would I be right in assuming that you can have both the refraction and keep the lower texture tiled properly with the top by reducing the height offset?