There is often amusement caused by seeing bottled water with a 'best before' or 'use by' date. Surely water doesn't go off?
It's a neat paradox that pure water doesn't go off - but water still should only be consumed relatively soon after obtaining it.
This goes both for tap water, once poured, and bottled water, once opened. In both cases it is best kept in the fridge and should be consumed within a couple of days. It's not that water itself can degrade. It's a very simple chemical compound, H2O - surely about the best known there is - with one oxygen atom and two hydrogen atoms. And the water itself will be exactly the same after a year. The trouble is that lots of living things need water, and like water as an environment in which to live. There are plenty of airborne bacteria, fungus spores and more that are floating around all the time. Once water is exposed to air it will start to accumulate these and, over time, you will get yourself a nice colony, some of which is likely to be bad for you.
If you have no choice but to drink water that has been kept for more than a couple of days (of course, unopened bottled water is fine up to its best before date), boil it for several minutes first.