I admire your dedication and tech savviness (it all sounds pretty complicated, cool you figured it out!)

Nice! Great job fixing that!

Here is a very common issue with Virtual Shadow Maps that you'll see pop up in UE5 games. Although generally speaking they're a big improvement over Cascaded Shadow Maps which games have used for a very long time.

(2/2) Of course, today I realised it's because I'm using raytraced shadows for my sun instead of the default Virtual Shadow Maps that UE5 uses. The shading model isn't set up for this, so the flat plane that has the water material applied was just casting a shadow on everything. This was far less evident when I would add a new light to test how it looked, because the new lights didn't have raytraced shadows enabled by default and didn't cause the water to cast a shadow. This wasn't a problem with low-light scenarios, all I had to do was disable raytraced shadow casting for the water material (bottom screenshot). The fix was a single check box.

This water isn't even visible in the scene I'm working on.

(1/2) I only touch my Unreal Engine stuff once every 6-12 months or something, so each time they've usually added some new feature that makes things much easier. This time it's a shading model specifically for water that handles light scattering and absorption, fake caustics, blah blah. This is very welcome because years ago I tried setting up my own water material for the Wrenhaven River - the river down the stairs in this area - and while it looked enough like water, it didn't really look good enough for me to want to include it in a scene.

Anyway, setting up the material is simple enough but I was getting such a sharp shoreline in low-light conditions (top screenshot) that I just couldn't fix. None of the Youtube or forum tutorials I could find mentioned this issue at all, and I had to force myself to take a break last night because it was pissing me off so much.