Some have also mentioned the fact that the spent fuel pools (not sure if they have them on site at any of these plants) also need to be kept cool and rely on circulating water (pumps) to keep them cool. If those pumps fail in several days they can get hot enough to turn the water into steam, and eventually they can get hot enough to melt down - only difference is they are not in a containment vessel, they will just release right out in the open.
Only they could simply run a fire hose in from the ocean, and one out back to the ocean. And I bet they would do it too, if it came to that.
Google the Arnie Gundersen who is fear-mongering the spent rods aspect in this WaPo screed.
Reactors are designed to contain a melt down.
IF there was a melt down it would make the clean up many times more complicated, but it would still be contained.