Well, I just watched the ending to Signs again on Netflix.
According to the news reports, somewhere in the Middle East they found a "primitive method" to fight back but they did not know what it was yet.
Now this could be:
1) Water (obfuscating the method for the surprise ending? in-story though it seems like it wouldn't be that hard to tell everyone *WATER*)
2) Faith (the holy water or whatever theory)
3) Maybe my idea of something in the water? (I'm not sure what exactly that could be though that would be considered "primitive")
Does leave it a bit open to interpretation though.