I heard somewhere that the only reason for the expiration date on the water was more for the "packaging" than the actual water.
The plastic in the bottle was created to begin to de-grade at some point so as to not fill up landfills if it doesn't get recycled. So, at some point in the future, if you hadn't opened it today, the bottle would have begun to break down and would not hold the water any longer.
As for drinking the water now, you'll be just fine.