Is the skylight an ambient light? Why can it lights up default water in closed space?

Hi guys I’ve made a simple quick test:
Use 1m Cube to build a room with a window, then put UE5’s custom water body with its default water (material)
I found that the skylight will not penetrate the wall to light up the room, it can hardly shine through the window and light up the room. However the water surface get lit significantly, and it is disproportionate with how skylight barely lit up the room.

The problem is:
Skylight can be effected by mesh, and it will cast shadow on mesh.
However, when it comes to the default UE5 water body, it works like an ambient light that ignores geometries around it.

That’s different from a typical offline renderer, where environment light will be blocked by every mesh/volume in the way of light rays.

the skylight is tied to the directional light I think.
You need a directional light for it to work properly.

Uhm I use Sun Sky blueprint so they should be linked by default.
I didn’t use landscape component, and I just use custom water body and the default Water_Material.
Still, sky light ignores walls around the water body and shine the water surface

I found a walkround: Turn down the scattering parameter in the default Water_Material, it will make the water more translucent.

However, it still does not solve the real problem of skylight lit the water surface without being blocked by geometry. It is just hacking.

Even if you use the sun bp
there still has to be at least 1 directional light active in the env mixer

the sky atmosphere, skylight, directional light all work together with exp H fog.
the skylight makes a sort of ambient light based on the directional light.

I got it

And I found that in Project settings, turn on the “Raytraced Sky light” helps a little bit.

However, the problem just got relieved, but not completely solved.
The large white specular area reduces its size, but it still shows at some angles where mirror reflection should not happens.