OKUMA, Japan—Nearly a year after the March 11 earthquake and tsunami sparked triple meltdowns at reactors here, the taming of Fukushima Daiichi has become in large part a quest to control water.

Foreign journalists on a tour of the Fukushima Daiichi compound Tuesday saw fields of squat, gray water-storage tanks; miles of orange, black and gray hoses; an AstroTurf-covered barge full of contaminated water; and white-suited workers huddled in a field preparing space for a new water container.

Water...