Watchdog commissioners instructed operator Tokyo Electric Power Co. on Wednesday to find where the water may be leaking from.
The plant was ravaged by the March 2011 earthquake and tsunami and has since struggled with leaks of water used to cool the reactors, hampering decommissioning efforts.
TEPCO has since May reported spikes in cesium levels in underground water collected from a coastal observation pit, a sign of contamination near the sea. But it has denied contaminated water reached the sea.
Watchdog officials said Wednesday that they have looked at the same data and suspect the water had over time reached the ocean.