On a completely unnecessary and didactic note, an alien physiology that would react to water like a high-pH acid is a pretty standard hard science fiction idea.
The problem with Signs is that aliens with (say) an NH3 body chemistry would want Earth at all (or could stand our climate, or even our O2-laden atmosphere). It would like if humans went to Venus, without suits, and tried to rough up whatever native life form could, in principle, live there. The rationale is so faulty, that the practical nitpicks aren't even the dumbest thing about it.
I once heard tell of a theory that the aliens weren't aliens, but were, in fact, demons. I have even seen this be referenced, in an unconfirmed fashion, as a theory that Shyamalan himself endorsed.
Now, it has some immediate appeal (the water is holy water, impliedly blessed by Mel Gibson's anti-Semitism; and demons are more prone to act like crack-addled burglars than an organized military). But it really doesn't fit into the film, with the spaceships (albeit invisible) and global struggle. So it's a bit of a failure, although you can see how, with the right tweaking, it could have been a movie about the Hellmouth opening up under Fucktown, Pennsylvania, instead of Independence Day, But Dumb