Is eau minérale masculine or feminine in french?

"hydro" is not a noun, it is a prefix, usually to a noun or adjective and does not affect gender.

If it is a prefix to a noun, the new noun formed will have the same gender as the original noun. If it is a prefix to an adjective, the noun the adjective refers to will provide the gender.

"Hydroélectricité" for example is feminine, because "électricité" is feminine.