We actually do use both in modern Greek, but "hydor" is mostly used in compound words. In ancient (but also in modern) Greek, "nearon hydor" means "young (i.e.

We actually do use both in modern Greek, but "hydor" is mostly used in compound words. In ancient (but also in modern) Greek, "nearon hydor" means "young (i.e. "fresh") water", and at least according to a greek professor whom I was watching on tv the other day, this phrase is where the modern "nero" originates from.