nic / coś + genitive
A similar construction as 'nic dobrego' can be met in modern Dutch. Of course, Dutch people do not realize any longer that the genitive is used there as the case system ceased to exist in Dutch, nevertheless they still use phrases like: 'niets belangrijks
', 'niets bijzonders
' or 'iets lekkers
So lots of water - dużo wody - is genitive
That's exactly this.