So if you have one of them in an olympic size swimming pool 50x25x2 meters, 2500 m^3, it'd need ~8 hours to evaporate the whole pool at full output.
If you assume decay heat as 1% of regular output (https://en.wikipedia.org/wiki/Decay_heat), you'd need to add (or have stored) ~3 m^3 of water per hour, or slightly less than a liter per second, to keep it from melting down.
If you assume an average of 2% for the first two hours, that'd be 8 MWh -> 12-13 m^3 for the first two hours, so a 5x5x5 = 125 m^3 pool (only considering the part above the "must always stay submerged" level) should be able to cool it for days.
I think _as long as the containment pool is intact_ (and you manage to SCRAM the reactor), this isn't going to be a major issue. But if e.g. an earthquake breaks the pool...
