Maybe Aliens Can Stand Water, But Only at an Extreme pH

Lots of organisms on Earth thrive at pH levels outside of the range of "neutral" water that humans prefer

pH, in case you don't know, is the measure of acidity or basicity of water. 14 being the most basic and 0 being the most acidic. This is a bit of an oversimplification, but for the sake of discussing a movie it should do

All living organisms that we know of require water in some form to carry out their biological functions. This is because water is the "universal solvent" and at a cellular level we need everything to be floating around in some kind of fluid for a cell to maintain it's structure and also be able to metabolize and what-not

The water we drink and use to bathe, which was what was used to fight the aliens should be at a pH at, or close to, 7, which we call "neutral".

It could be that aliens do use water, like every other known living thing, but have evolved so that a pH of 7 is corrosive and toxic

The main problem with this is that there is humidity in the air. Water vapor exists in the atmosphere at certain small concentrations just about everywhere. So it would be like they were walking in an environment filled with a toxic, corrosive gas. But maybe they can tolerate very small concentrations of water

I think Shyamalan drew inspiration from Wells's War of the World's, in that everyday objects (microbes in the case of WotW) are deadly to the alien invaders. Water is definitely sillier, but not totally out of the realm of possibility

Another flaw with the idea of water being toxic to aliens is that large modern farms with lots of crops should have fairly sophisticated irrigation systems, so there should be way more moisture in the crop fields relative to the surrounding atmosphere. So crops fields would probs be among the last places that these naked aliens would lurk. But Shyamalan is a writer/director, I doubt he knows much about farming. Oh well