New reports indicate that attempts to drop water on damaged Japanese reactors have been unsuccessful. The USA has a robot helicopter water-bomber which might be safe and effective.
In January 2011 U.S. Army Natick Soldier Research, Development and Engineering Center (NSRDEC) is reported to have successfully tested with an unmanned helicopter air-dropping a 4,400lb load. The helicopter used was a modified version of the Kaman KMax, which is also used for fire-fighting around the world (including in Australia). The aircraft can carry a water-bucket as its load and release the water under remote control.
It appears likely that the Kaman KMax aircraft could be used to drop water on a nuclear reactor. This could be done with more precision than a manned aircraft as there would be no risk to the operator and the aircraft could hover precisely over the target. The aircraft could operate for hours at a time, then needing to land for a few minutes to refuel.