Exactly, the “Enhance HDR” node group just does what it says: It increases the contrast of the environment HDRI to e. g. get sharper shadows from sunlit environment scenes. And it also allows to easily use a low-res HDRI for the lighting and a high-res LDRI for reflections and camera (or to just have different brightness values for lighting, camera and reflections with the same environment texture). But that node group has nothing to do with the Snell’s window effect.
Top: Just HDRI connected to backgroud node, strength 6
Bottom: Same HDRI, but with Enhance HDR node group, strength 3
I have this node group in my startup scene - sorry for the confusion!
And I wouldn’t say that the seabed mesh has no effect - it significantly darkens the areas outside of the “window”, which I find nice.
The water mesh is a cube in order to have the option to add “volume” shaders later, as I can’t really imagine an underwater scene without. But of course, do this in your scene as you wish.
Anyway, I hope you can sort this out!