Help needed - 2 blender files, the same node groups, the same scenes - only 1 works

Being newbie and such I am gathering useful node combos from tutorials. The group of question is the 1st part of a tutorial about the bottom of a shallow lake. I made the group and it worked as intended, so I saved it as a library.blend. Then started to create the scene water.blend and appended the same group - which this time stopped to work at all although the preview window shows the correct texture. I spent literally hours trying to figure WTF is going on and still cannot find the reason. So please look at both files and tell me what mistake I am doing.water.blend (1.93 MB) library_nodes.blend (1.88 MB)

In ‘water.blend’, the Bottom mesh has two materials: Water and Material.001. The group you mentioned is used in Material.001, but this material is not assigned to the mesh. Instead, it is the Water material that is assigned to the mesh; and this material has a glass node setup. Glass is for the most part transparent, which is why you don’t see anything (with the World background being black).
To solve the problem, assign ‘Material.001’ to the mesh: go to Edit mode, select the whole mesh, then select ‘Material.001’ and click on Assign.

Arghhhh… Silly me! Thanks a lot.
I often select the objects in the outliner and sometimes the object name becomes white but it is still the previous object which remains selected and thus causing a mess.