Er, the problem is precisely that a planet two-thirds covered with water can't support such creatures, any more than one two-thirds covered with sulfuric acid could support humans.
Look, Signs used "aliens" as a plot device. Given that, it's going to have a non-believable plot!
I'm not inclined to excuses for incompetence (and, yes, that word is applicable to a writer who creates a "non-believable plot").
Um, why not? What about the remaining one-third? Again, say you have no choice. You see a planet. Two-thirds of it is covered with sulfuric acid; the remaining one-third is usable, arable land (and the atmosphere is fine for your lungs). You see no other planet remotely usable. You don't land there "because it's two-thirds covered with sulfuric acid"?
Heck by your logic humans shouldn't live on earth either. As you say, the earth is two-thirds covered with water. Humans cannot live on water!! Humans can swim for short periods but they cannot breathe underwater and would not be able to subsist were they to spend their whole lives on the water!!
Too much ocean - We better get outta here!!!1
The credibility of a fictional plot is based on whether or not the story logic avoids gaping holes once the basic premises of the fictional universe are accepted
Fair enough but Signs had no such holes, you're obviously bending over backwards looking for them, and doing an illogical job of it. See above.