I believe the flavored water has splenda (sucralose).
There have been animal studies (including dogs) to test the safety of sucralose for humans and so far it has been found to be non-toxic and non-carcinogenic. Most of the sucralose is excreted in the feces & urine. They did find that high doses caused Gastrointestinal upset in rats as sucralose isn't digested. Here is the article I found about human and animal testing of sucralose
http://www.kon.org/urc/frank.html
But then I found this website that claims the following about high doses of Splenda:
Shrunken thymus glands (up to 40% shrinkage) (EO56)
Enlarged liver and kidneys. (EO57 & E161)
Atrophy of lymph follicles in the spleen and thymus (EO51, EO56, EO151)
Increased cecal weight (E151)
Reduced growth rate (EO57)
Decreased red blood cell count (EO55)
Hyperplasia of the pelvis (EO57)
Extension of the pregnancy period
Aborted pregnancy (E134)
Decreased fetal body weights and placental weights (EO32)
Diarrhea
http://www.holisticmed.com/splenda/
All the Dasani flavored waters also contain magnesium sulfate, potassium chloride, salt, and natural flavor. Too much salt is bad for dogs, although it's a very small amount in bottled water.
So I guess it all comes down to if you think artificial sweeteners are safe or not. If you give it, I would only give a VERY small amount. Our yorkies are so small and too much splenda would most likely cause an upset stomach.