Can see through terrain -- why?

I have created a simple scene with a terrain that has a lake (I used asset WaterProDaytime for the water) in which a cube falls and floats.
When the cube sinks in the lake, with the camera on a side I can see the cube, as if there were a hole in the terrain.

Why is that and how can I fix it?

See pictures below (in the first one you can see the cube through the terrain when the camera is on the side. In the other pic with the camera from the top, you can see that of course there’s no hole in the terrain when removing the water)

It’s because that is the backface of the object, i.e. the negative of the object’s surface normal. By default, shaders in Unity don’t render or “cull” backfaces to create a moderate performance boost. Without doing this it would almost be like rendering every mesh in your scene twice. The only real way to fix that would be to disable culling in the terrain shader or use your own.

To download the built-in shaders;