hydr, hydro, hydra
These prefixes are related to water. For example: hydroelectric power is energy made by producing a dam in which water flows through turbines, turning electric generators which then produce energy.
"Hydra," in Greek mythology, was a name given to a mythical water serpent. So, it would seem, "hyd-" or "hydro-" have taken on meanings, as prefixes, closely associated with water.
The prefix is very common in English, and often will be seen in drug names or chemicals, where elements or compounds are bound with water as an ingredient.
Word List
hydraulic
hydroelectric
hydrofoil
hydrogen
hyrdrant
hydroponics
hydrometer
hydroplane
hydration
dehydrated
hydrophobia
hydrotherapy
hydrosphere
hydrodynamic
hydrocephalics
hydrochloride
hydromechanics
geohydrologists