(So let's just hope they're abundantly stupid like the aliens in Signs and never take into account the fact that if water kills you on contact, maybe you shouldn't invade a planet covered in mostly water, inhabited by beings made of mostly water. HTF did that movie ever get past the script stage?)
Use a little imagination, say the beings in Signs were actually biologic helper-constructs and that the water vulnerability and low IQ were checks put in place by their creators to prevent them from pulling a planet of the apes on their asses. The things are mass cloned from eZEmatic constructor nanites for zero cost anyways, who cares if they get fucked up.
For a sci-fi nerd, and I assume you are if your still thinking about Signs seven years later. You seem to have taken a lot of preconceptions about alien species to the theater with you. Try to have an open mind. Remember, just because a good director has to make a story plausible within the bounds they define doesn't mean they have to spoon feed the viewers everything! The water susceptibility and IQ weren't important to the story so they were left up to us.
When I left you, I was but the pupil. Now, I am the master. - Darth Vader