Signs - It's not just water, it's water coupled with faith - i.e. holy water.
Except, you know, the literal post above you quoted the writer/director and he didn't say that.
His point is that it's not relevant to the story. Make up your own reason.
Why do thieves steal money from banks protected by security guards?
Why do humans snorkel when they could drown?
Maybe the aliens only react to water of a certain PH level that they identify by placing "land here" signs.
Who cares? They're a MacGuffin and their biology is not relevant to the story.
1. Cop out for him not thinking his plotline though. I'm not willing to just ignore common sense. And I frankly shouldn't have to make up stuff to fix a plot hole. That's the writer, editor, and director's job.
2.
Because banks have a **** ton of money in them and therefore it's worth the risk/challenge.
3. Humans aren't moving to a snorkel only to survive planet. And I didn't get the sense here that these aliens were just taking a vacation.
4. Wat
5. I care. It's a really stupid plot. Ruined, at least for me, the several positives (tension, acting) of the movie.