Not that Zircalloy won't dissociate, but you're correct, don't discount the water either.
At one point during the Windscale fire they tried to pump down liquid CO2, but the reactor face was so hot it was ripping the O2 off the molecule. They next tried to pump down water - there was a very real risk that it would rip the water apart as well, feeding the fire and potentially forming a hydrogen bubble and exploding. However they were able to deliver it in greater quantity than the CO2, and succeeded in cooling the fire.
At one point during the Windscale fire they tried to pump down liquid CO2, but the reactor face was so hot it was ripping the O2 off the molecule. They next tried to pump down water - there was a very real risk that it would rip the water apart as well, feeding the fire and potentially forming a hydrogen bubble and exploding. However they were able to deliver it in greater quantity than the CO2, and succeeded in cooling the fire.