Hydro-Man, Fluid Felon is a rare Legendary Creature — Elemental Villain card from Through the Omenpaths (OM1), collector number 27.
Whenever you cast a blue spell, if Hydro-Man is a creature, he gets +1/+1 until end of turn. At the beginning of your end step, untap Hydro-Man. Until your next turn, he becomes a land and gains "{T}: Add {U}." (He's not a creature during that time.)