The expiration date on water bottles is for the bottle, not the water
There are several reasons why water bottles come with an expiration date. The main one is government bureaucracy: Water is a consumable food product, and as such, it is subject to laws requiring expiration dates on all consumables, from bologna to lemonade.
Besides that, the expiration date on bottled water has certain benefits for the manufacturer.
Although water, in and of itself, does not go bad, the plastic bottle it is contained in does "expire," and will eventually start leaching chemicals into the water. This won't necessarily render the water toxic, but it might make it taste somewhat less than "mountain spring fresh." If consumers contact drink companies to complain that water they bought several years earlier tastes bad, the bottlers can point out that it's their own fault for not drinking it by the expiration date.