I make my own ammo. I can make more. If ammo is still scarce when I run out of primers and powder, I will have led a long life.
Water? Perhaps in urban environments where you only have to transport it short distances. If you are the only one with water in your area, it won’t matter how much ammo you have.
The survivors will not be in urban areas. They will be in rural areas with water, land and game. Urban dwellers will simply be using up the few months of supplies they have stored. They will probably be the first ones killed in any case.
Water is an absolute requirement, but a lousy commodity to hoard or transport when fuel and spare parts for vehicles are not available. Live near your water supply.