Use of seawater in and of itself is not a deal breaker. Saltwater corrosion occurs in susceptible materials over a relatively long time. Materials used in the pressure vessel and vessel internals are very resistant to most corrosion mechanisms. If they want to salvage these reactors they will have to go through a cleanout procedure to remove residual minerals, sodium chloride, and other things, but chemical engineers know how to do that. The cleanup of the K-25 GDP in Oak Ridge was a greater challenge in many ways. You had things building up in that diffusion cascade for decades that had to be flushed out.
Lets calm down and see how this plays out. In the end, it is almost certain they will restart units 5 and 6. Unit 4 had the SFP problem and they will have to get that settled down. The Unit 4 reactor was probably not too badly damaged and probably can be salvaged. Units 2 and 3 had fuel in the core and they will have to get in there and check the extent of the damage prior to making a restart decision. Unit 1 was the oldest and had core damage so of all of them this is the one they will probably retire, most likely doing a SAFSTOR, like we did with TMI-2.
Lets calm down and see how this plays out.
(1) I'm quite calm, Note that I have been asking question in order to get information, not yelling chicken little.
(2) And it was a rep from Tokyo Electric on NHK last Wednesday that said once salt water was used the reactors would probably never be restarted. I was just going by that as gospel.
As for 5 & 6 as I said I looked it up and they were in cold shutdown a the time of the quake so the salt water might not be an issue in that case.