Back to Genetive.
When someone enter in the shop I say to him:
Вы хотите / Вы хотели бы воду/пиво/вино?
They told my that is better to say: Вы хотите / Вы хотели бы воды/вина/пива?
The problem is that I don't understand whether this is Roditel'nyi or just Iminitel'nyi / Mnozhestvenoe chislo.