Some problems with just adding water when the temperatures are too high:

Steam explosions. Happens if you heat water very rapidly in an enclosed cavity. Water and very hot metal is a very volatile mix if the rapid expansion of steam has nowhere to go. (I remember that one liter of water would turn into 1200 liters of steam in an instant if you put it in direct contact with molten metal).


Steam will have to be vented to the outside air. Hopefully, the steam itself won't bee too radioactive (at least not for too long) and hopefully it won't drag highly radioactive aerosols (particles small enough to be airborne) with it


Chemical reactions between the water and components in the reactor control rods and/or fuel rods that leads to formation of hydrogen. Mixed with oxygen it is very explosive. There has already been several hydrogen explosions, if I am not mistaken.