"Hydro" is a prefix that means "water". So, depending on the context, you might consider antonyms like:
* "A" or "An": For words like "hydrophobic" (fear of water), the antonym could be "hydrophilic" (attracted to water).
* "Dry": This is a general antonym for "water" and could work in some contexts.
* "Terra": This prefix means "earth" or "land" and could be used in some contexts, such as "hydroelectric" vs "terrestrial energy".
To provide a more accurate antonym, please give me the full word or phrase that "hydro" is part of.