If you just extend the metaphor slightly and have tap water mean 'comes from a tap' rather than 'comes from a municipal supply' it still works. The supermarket

If you just extend the metaphor slightly and have tap water mean 'comes from a tap' rather than 'comes from a municipal supply' it still works. The supermarket that currently has thousands of individual plastic bottles could have a big tank with a tap on it. It sounds more like a trust thing. You need to know that the water is safe. If you know it's safe, you're good.