The show has loopholes. I have serious problems with Aliens coming to the Earth. There's no reason why any alien species with the technology to traverse the stars would need our pesky world.
Yeah, the whole "they're here to steal our water" thing never made sense. There's millions of times more water (in the form of ice) in the Jovian moons, Kuiper Belt, scattered disk, and Oort Cloud than there is on Earth, and it's a lot easier to get to without the need to fight the gravity of both Earth and Sol. Not to mention all the ice that could be found in every other system between here and Sirius.