I'll see what I can do to improve this. If the subsurface media is very dense, for displaceable objects you should try to use a different shader that has a non-scattering subsurface (e.g. Default Shader).
At the moment (v4.5), displaceable objects with displacement create difficulties for the subsurface effects in the Glass Shader and Water Shader, and there are situations where it doesn't work well, depending on the render method, scale of the displacement and the ratio of micropolygon size to the scattering distance.
In the Standard renderer (as of v4.5)
The subsurface volume is trying to approximate a subsurface scattering effect without actually scattering the light, but this only works when the conditions are right. In order to appear soft and maintain the illusion of scattering when the the surface is displaced, it ignores the displaced normal and uses the undisplaced surface normal for illumination. Unfortunately this means it doesn't look right for dense media (or displacement that's large relative to the scattering distance). This render mode also relies on the object to have shadows disabled if you want the parts facing away from the light to receive light (as in the default Lake setup).
In some situations this would look better if it used the displaced normal so I might add that option in future.
In the Path Tracer (as of v4.5)
"Hard surface approximation": This has some of the same limitations as the Standard Renderer (i.e. choice of surface normal). Perhaps I should add an option to choose which surface normal to use.
"Subsurface scatter towards normal" and "Subsurface scatter in all directions": These options works well if the micropolygons are small enough relative to the scattering distance, but they fall back to the hard surface approximation (above) when the scattering point is too close to the surface relative to the micropolygon size. This is more likely to happen with high density of the subsurface medium, but it's also more likely to happen when the surface is far from the camera (where micropolygons are larger). This fallback is necessary because the scattering rays and the visible surface renderer don't use the exact same micropolygons (because of the Ray Detail Multiplier), so fully ray-traced scattering doesn't work at small scales (although it works fine for non-displaceables). I want to unify the micropolygon engines in future, and this would solve the problem.
I think it would look better if the fallback used the displaced surface normal, so I'll look into that.