Of all the liquids you can buy, bottled water is the one where the package design is the most important way to distinguish the product. After all, the contents are not only boringly colorless, but they're all essentially the same stuff: two parts hydrogen, one part oxygen. Not to mention that people don't typically pore over a water purchase the way they would with, say, a jar of tomato sauce. You run into the convenience store because you're thirsty and grab whatever looks good, without really agonizing over which spring it's from.
Package Design Magazine's Bill Wynkoop takes a look at some notable package designs in the "ultra-competitive" category here.