So basically your comment is "this movie was awful, because it wasn't what I expected it to be."
Signs isn't hard science fiction, which you seem disappointed with. It isn't about exploring the nature of aliens, or the possible future repercussions on society, it's an alien invasion seen from the eyes of a small family. It's about how an invasion would look to the everyman, the normal people, rather than the heroes and politicians that virtually all other alien invasion movies focus on.
It was also created to be a movie where the nature of the aliens was in doubt. Virtually all other alien movies make it clear from the very beginning whether they are about invading aliens or friendly aliens.
Besides, in the movie they made it quite clear that the aliens only landed in dry spots, and had come for a quick snatch and grab. People only discovered that water was dangerous to the aliens by accident, in only a couple of areas of the world.