While bottled water typically contains an expiration date, it does not actually go bad. However, over time the taste of the water may change due to the plastic bottle's chemicals leaching into the water.
The taste of the water does not indicate that the water has gone bad. In fact, the expiration date stamped on the bottle is usually nothing more than a legal requirement. As with all food products, once the seal on the bottle of water has been broken, the water should be consumed or used within a reasonable amount of time to prevent the growth of algae and bacteria.