I enjoyed Signs I really did, I mean this on the linear level - I enjoyed Mel Gibson's acting, I enjoyed the mystery and intrigue surrounding the aliens. Especially when they were at the house at night and on this basis Signs was for all intrinsic purposes a good film.
However, if you watch the film with any semblance of sanity and intelligence it eventually becomes obvious that an Alien civilisation which can transverse the indeterminable vastness of the universe with the added capability of a fully blown military invasion tagged onto the end of this gigantic journey - that these Aliens, these highly educated beings might have noticed, nay, might have detected long before sight of Earth those two really, really big pools either side of the continents. I.e. The Atlantic and the Pacific, now, dare I say these beings will probably have come across this abundant substance somewhere else, you know, on their endless voyages through space and this particular liquid substance, known as water, was, well you know....highly acidic to their skin.
The same substance which, any intelligent being would probably call our planet, "Ah look, theirs water." Not...."Aha, Earth, no water their boys...accept, those two really big blue pools of water."
M. Night Shyamalan.