Depending on the soda they have additives that have a variety of effects that water doesn't.

Huge influxes of sugar will cause long term health issues, a risk factor for diabetes for example, and weight gain which has it's own issues.

Caffeine has an effect on your brain chemistry, generally your body over a long period of time will "adjust" to the presence of caffeine meaning that you will require caffeine to feel normal. You are addicted to caffeine.

They also have other additives that essentially dehydrate your body. So you will intake water, quench thirst in the short term, but you will be thirstier after 30 mins than you would be with water. Meaning you drink more.

The long and short of it is that Soda's (even diet) are basically designed not to hydrate you, but to make you want to drink more Soda.

For the most part your body doesn't care about where it gets it's water. About 17% of your water intake comes from food. The issue is the other compounds in soda.