General words:
* Hydrology: The study of water and its movement on, above, and below the surface of the Earth.
* Hydrophobic: Repelling or tending to repel water.
* Hydrophilic: Having an affinity for water; readily absorbing or dissolving in water.
* Hydrous: Containing or composed of water.
* Hydroponics: The cultivation of plants in a water solution without soil.
* Hydrostatic: Relating to fluids at rest.
* Hydrodynamics: The study of fluids in motion.
Specific words:
* Hydroelectric: Relating to the generation of electricity by water power.
* Hydrocephalus: A condition characterized by an abnormal accumulation of cerebrospinal fluid in the ventricles of the brain.
* Hydrant: A pipe with a valve that allows water to be drawn from a main pipe.
* Hydrogen: A colorless, odorless, highly flammable gas.
* Hydrotherapy: The use of water for therapeutic purposes.
* Hydroxide: A compound containing the hydroxyl ion (OH-).
* Hydrometer: An instrument for measuring the specific gravity of liquids.
* Hydrofoil: A wing-like structure that lifts a boat out of the water at high speeds.
Prefixes and Suffixes:
* Hydro-: A prefix meaning "water".
* -hydric: A suffix meaning "pertaining to water".
This is not an exhaustive list, but it gives you a good starting point for exploring words with the root "hydro".