I still believe the great fan theory about Signs about the aliens being actual demons to be by far the best fan theory for that movie. It sadly makes the most sense and would explain how the cups of "holy water" managed to successfully melt the aliens with excessive ease.

Last I knew about Signs the literally melted to death alien there literally no other reports in the movie about water somehow killing the aliens. Just that it all ended just as fast as it began. Begging the question "Why bother invading the Earth if you're not going to put much effort into it?". It's been a bunch of years since I last watched Signs so I could be wrong about the end of the movie.