I assume that by describing these drinks as “unsweetened”, you mean they do not include artificial or natural sweeteners.
Carbonated drinks can contribute to your body’s hydration. The potential for other harms and benefits depend on other ingredients in the beverage, such as caffeine and possibly phosphoric acid.
There was a concern that phosphoric acid could contribute to bone demineralization but studies have not concluded this is the case. Side effects from excessive caffeine consumption include anxiety, restlessness, rapid heartbeat and/or palpitations, insomnia, etc.
Seltzer/soda water and carbonated waters such as Pelligrino and Perrier are not considered to have any harmful side effects and their benefits would be the same as drinking any still water or tap water that has some minerals in it.