One of those things that you think you are doing well by drinking water. Just water, right? Think again.
Lots of oil went into the bottle, then more to get it to you. And even if you recycle it, this only saves a little bit of carbon since there is still all that energy that will go into making it usable again.
Use a recycled bottle, then use tap water. Save you big bucks, and it is better for the environment. And since many of those bottled waters use tap water anyway, you're just as well off in the end health-wise.