For soda makers, that their product's base is water means they can market or deflect around hydration and thirst.
But there is no hydration emergency.
If the body says drink, definitely do so, and water of course is your best bet.
Yet the world around has a bad habit of making it seem all the more complicated. Recently the New York Times wrote about "surprising ways" to stay hydrated. It covered the new "hydration index" study funded by the European Hydration Institute (EHI). The index ranks drinks according to their hydration abilities. Among other findings it reported that regular soda is as hydrating as water.
The EHI might be fairly described as the Global Energy Balance Network of hydration as it was founded by Coca-Cola at a cost of just over $7 million USD, and the study's lead author, a prior consultant and speaker for Coca-Cola, is the chairman of the EHI's scientific advisory board and the vice-chair of the EHI's board of trustees.
College Humor recently put out a great debunking of the hydration emergency. Have a peek!
All this to say that soda benefits from the hydration emergency message as it healthwashes their product in a way that leads people to think of soda as something more than just liquid candy.
So yes, definitely drink if you're thirsty, but drink water, not candy.