Mr. Adventure wrote:
Wasn't it more like there was something in the water? IIRC the little girl wouldn't drink the water because she thought there was something wrong with it which is why she left all those glasses of water around the house.
No, that was just what "God" made the little girl think so that she would leave glasses of water all over the house that could be spilled on the alien when they were fighting it in the living room. I think she uses a couple of different reasons for not liking it, from it tasting funny to it being full of "microbes," which of course there are in everyone's drinking water every day.
It would imply a global change in the content of the water for your idea to work, since it was mentioned on the radio that they were killing the aliens with water in the Middle East and people were gathering around bodies of water because the aliens seemed to be avoiding them. I think someone would have noticed if the microbial content of water around the world had drastically and uniformly changed.
Plus, even if it were just the water around their farm, he waters his crops from the same water source, so walking naked through those water-covered fields of corn in the constant mist depicted should have been agony for the aliens. I could only imagine what those poor aliens harassing kids at the birthday party in Brazil so close to the world's largest rain forest must have gone through. I guess also they checked with Stormy Biiblebrox the alien meteorologist for the global Accuweather Doppler 5000 forecast to avoid the hundreds of places on Earth where it would be raining at any given time.
Additionally, the water damaged the alien's skin instantly, like it was acid. Even with extraterrestrial biology, it stretches logic (even more so than the water) to imagine that microbes would act that fast or have that effect.
The movie (admittedly unevenly) implies that God doesn't so much take direct action on the big stuff, but rather gives you little hints, nudges and personal traits to point you in the right direction. If not, surely he could have simply made it rain and saved everyone a lot of trouble. By "unevenly" I mean that I guess causing a poor guy to fall asleep at the wheel after a hard day at work helping animals so he could cut another faithful follower's wife in half with his car so she can deliver a vague message about the now faithless man's brother seems like taking pretty direct action to me, just in a completely ridiculous and roundabout way. So does making his brother unable to avoid swinging at every pitch, his daughter paranoid of water contamination, and giving his son serious asthma. The movie seems to imply that these are gifts from God, which then calls into question why he couldn't intervene in stopping the aliens before millions of people were killed or kidnapped for who knows what purpose.