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?
maybe you find something usefull here https://www.duolingo.com/comment/12825287
why is this genitive and not instrumental as it describes something?
It's 'partitive', one of the usages of Genitive. Basically, "of" automatically suggests Genitive. Any container of something will take this 'something' in Genitive.
So how would I say, "A big water bowl"?
Apparently, the same. Added now.
Even if it's an empty bowl intended for water? (Maybe you're buying one for a pet?)
Hah, true. That's "miska na wodę".
Great! Thank you Jellei :)
Duża takes feminine declension because of miska or woda ?
Of "miska". "a big bowl", or "a big bowl of water"... that makes no difference for the word "duża".