I like the fan theory that they aren’t aliens at all, they are demons. The broadcast at the end that reveals their vulnerability is cut off and nothing ever definitively points to them being extraterrestrial. In this theory they aren’t weak to water itself, but holy water which had been inadvertently blessed by the dad who used to be a priest (a bit of a stretch but if I remember right I think we see him mouthing a prayer at one point).

I don’t think that was the intent but if you rewatch the movie with this in mind it’s a pretty interesting way of explaining some inconsistencies in the film.