Soda water is not unhealthy if it's plain and free of added sugars and artificial flavors.
As long as the soda water is plain soda water it's healthy to drink and still hydrating like normal water would be.
The carbonation of the water is not unhealthy but any added sugars and artificial flavors can make it unhealthy.
Although drinking too much soda water could lead to gas or bloating in some people due to the carbonation.
Actual soda on the other hand is unhealthy when consumed in excess.
Actual soda when referring to soda pop is high in sugar and calories and drinking too much soda pop can lead to weight gain, tooth decay and even an increased risk of diabetes, heart disease and other chronic health conditions.
Even diet soda has been linked to some negative health effects.
Soda pop can also be hard on the kidneys when you're drinking too much of it.
As long as the soda pop is consumed in moderation and you're drinking other healthy drinks like water, tea, fruit juices, vegetable juices etc you can remain healthy when drinking soda pop in moderation.