When I first saw those cute bottles on Gossip Girl I had to look them up.. "Vitamin Water", with a sleek website and bottles that state: vitamin + water = all u need.
They will also have a vitaminwater beach thing going on at the Cannes film festival this year and it looks super fab! Link
But Vitamin water is basically soda (not diet soda) fortified with vitamins.. Think "still ornamin C" NOT "healthy water". It contains juice extracts, sugar, Vitamins (A, B's and C) and other combinations of antioxidants, zinc, st johns wort or ginseng according to the flavour you choose.