It's not that water kills the aliens. It's that tap water kills them. Tap can be very poisonous to living beings. The movie gives it out by hints -- Bo says that her water is contaminated. It's probably the tap that murdered the aliens. The aliens survived on a planet filled with water for 30 years (according to the film). The aliens managed to kill a lot of people. Merrill said that "a lot of people died." It's the tap that killed the aliens. The family was blessed. In the right place, at the right time.