Sorry that i keep forggeting but can somebody link me to somwhere that explains all the different endings as well as meanings and when to use them?
It's 'partitive', one of the usages of Genitive. Basically, "of" automatically suggests Genitive. Any container of something will take this 'something' in Genitive.
Even if it's an empty bowl intended for water? (Maybe you're buying one for a pet?)