This one is a classic. In the famous TV mini-series, V, our hero, Mike Donovan (Mark Singer), is inside the mother ship and his Visitor friend, Martin, explains to him why the Visitors have come: they traveled through interstellar space to steal Earth’s water. That’s kind of interesting because if they traveled from another star to Earth, they passed through the largest reservoir of water in the Solar System: the Oort cloud and the Kuiper belt.
Most estimates agree there is more water off Earth than on it. But let’s say you just need the hydrogen from the water for your fusion engines. Hydrogen is the most abundant element in the Universe, so much so that physicists have designed interstellar spacecraft on paper that can scoop up the disperse hydrogen of interstellar space, cram it down a giant metal throat, ignite it in a fusion reaction and propel the ship between the stars at relativistic velocities.
This is an example of grafting Earthly problems (the scarcity of fresh drinking water) onto aliens.
 Someone wrote in saying, “Wasn’t that just a ruse?” The water part, no. The Visitors came saying they needed certain chemicals which were flown up to the mother ships then dumped overboard. Their real goal was water and “food”. Guess who they were having for dinner? 🙂