Thanks for pointing out my error tactfully by rewriting "bottles of water" without the options I had in brackets. I had overlooked that one letter, 'z', in the statement which tells us that bottles is in the plural here.
"chupa za maji" = bottles of water
"chupa ya maji" = bottle of water
I'm pretty convinced (while waiting for Machieng) that this use of the genitive can only mean a bottle with water in it, and not a water bottle, meaning a bottle for containing water.