From the "For What It's Worth" Department:
I read somewhere that the plant was nearing the end of it's design lifetime, and that they were going to start decommissioning some of the units starting next year. That may have played into their decision making about adding ocean water.
Besides salt, there are many other minerals in the ocean (consider that every time it rains, the runoff and everything it picks up eventually ends up in the ocean). One of the problems with introducing water that is not distilled or demineralized is that it can cause serious corrosion. Another thing that can happen is that the impurities in the water can become activated. Activated means that it has been exposed to radiation, and that in turn makes the impurity radioactive.
In every power plant (nuclear and fossil), the conductivity of the water (impurities increase the conductivity of the water) is closely monitored. An increase of conductivity means an increase in impurities in the water. Conductivity is measured in mhos (the opposite of ohms which is resistance).
Anyways, the impurities in the ocean water has probably caused extensive corrosion problems already (increased heat accelerates the corrosion process). And, the impurities in the water has surely become activated. So, those are some of the reasons the unit(s) will never run again.
Of course, they still haven't determined the extent of the damage so far (as far as I know). I'm sure that as they discover more about the condition of the units, further explanation will be available.