Merriam-Webster's Collegiate Dictionary:
hydrolyze
hydromagnetic
hydromancy
hydromechanical
hydromechanics
hydromedusa
hydrometallurgical
hydrometallurgist
hydrometallurgy
hydrometeor
hydrometeorological
hydrometeorologist
hydrometeorology
hydrometer
hydrometric
hydromorphic
hydronic
hydronically
hydronium
hydropathic
hydropathy
hydroperoxide
hydrophane
hydrophilic
hydrophilicity
hydrophobia
hydrophobic
hydrophobicity
hydrophone
hydrophyte
hydrophytic
hydroplane
hydroponic
hydroponically
hydroponics
hydropower
hydroquinone
hydrosere
hydrosol
hydrosolic
hydrospace
hydrosphere
hydrospheric
hydrostatic
hydrostatically
hydrostatics
hydrotherapy
hydrothermal
hydrothermal vent
hydrothermally
hydrothorax
hydrotropic
hydrotropism
hydrous
hydroxide
hydroxide ion
hydroxy
hydroxyapatite
hydroxybutyric acid
hydroxyl
hydroxylamine
hydroxylapatite
hydroxylase
hydroxylate
hydroxylation
hydroxylic
hydroxyproline
hydroxytryptamine
hydroxyurea
hydroxyzine
hydrozoan
hyena
hyenic
hygeia
hygiene
hygienic
hygienically
hygienics
hygienist
hygr-
hygro-
hygrograph
hygrometer
hygrometric
hygrophilous
hygroscopic
hygroscopicity
hying
hyksos
hylozoism
hylozoist
hylozoistic
hymans
hymen
hymenal
hymeneal
hymeneally
