The files retrieved from the Climate Data Store (CDS) for 'Water quality indicators for European rivers' and 'Water quantity indicators for Europe' datasets contain catchment data instead of data gridded on a lat/lon grid. Moreover, the file itself does not contain all required metadata.
To properly interpret the data in the files it is necessary to have access to the GIS shapefile (corresponding to this particular dataset) that describes the catchment areas.
The shape files could be found by following the DOI_link given in the netCDF files. For convenience that is:
The shape files will be included with the next version of the "Water quality indicators for European rivers" dataset due for delivery in January 2021.