I saw two possible explanations for how the aliens react in Signs:
1) It's actually a coming of age ritual: "You will not be a true adult until you go down to the Planet of Watery Death naked and barehanded, and return with the head of one of the Water Monsters"
2) They aren't actually aliens at all, they're DEMONS. After all, they never self-identify as aliens, and they don't act like technologically aware aliens. They do however, act like imps or demons. Which explains why splashing them with water works- it's actually sanctifying them.
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1) It's actually a coming of age ritual: "You will not be a true adult until you go down to the Planet of Watery Death naked and barehanded, and return with the head of one of the Water Monsters"
2) They aren't actually aliens at all, they're DEMONS. After all, they never self-identify as aliens, and they don't act like technologically aware aliens. They do however, act like imps or demons. Which explains why splashing them with water works- it's actually sanctifying them.