That's a really good summary. Ebert would miss details, but I think his assessment of Walkabout is spot-on. It works brilliantly on at least three different levels. The first is as a simple survival story. The second is as a contrast between nature and urbanization, the monotony of a structured civilization versus the fantasy-paradise of the uninterrupted outback. As Ebert states, it would be easy to read the film as a condemnation of civilized life, but Roeg's direction is more complex than that. This leads to the third level on which the film works: as a study of communication in all its forms. There's the lack of communication between the father and his wife and children that contributes to his suicide. There's the lack of open communication between the girl (and boy) and the white man at the end of the film who works in the abandoned mining town and does not seem to comprehend or care about the children's unimaginable plight.
And finally, there's the difficulty the girl in particular has in communicating with the Aborigine and reciprocating his advances. I think Ebert understood that they were capable of same basic communication over water, but his broader point is that their failure to communicate adequately led to his suicide, just as happened with the father. It's fairly obvious that the girl is afraid of the Aborigine raping her - that she's bright enough to be aware of this possibility and afraid of it - but her suspicions don't change the fact that there's a fundamental lack of adequate communication between the two teenagers that dooms their relationship (sexual or otherwise) and even contributes to her future dissatisfaction with a humdrum civilized life.
Speaking of a lack of communication as a barrier to sex (or perhaps sex as a barrier to communication), I actually like the interpretation proposed earlier in this thread, that the father might have been sexually abusing the girl. Roeg definitely implied something off-color, what with the father's lingering gaze and all those prolonged shots of her in the nude and putting on her underwear. It's as if the camera has sex on its mind before even the Aborigine does. Certainly, the sexualization of the girl - the male gaze of the camera - contributed to a sense of foreboding and served as a constant reminder to the viewer that the girl was not physically safe so long as she was abandoned in the outback.
So I think Ebert was spot-on in his assessment of the film, which speaks to something deeper than an inspirational survival story or a parable about the noble savage. Certainly those are accurate surface interpretations, but plunging deeper, the target of Roeg's criticism is not so much industrialization but the failure to communicate, especially when sexual/romantic motives are at play.

Ebert (when he was still alive) often missed or misunderstood crucial details of the films he was reviewing. I chalked this up to him professionally having to watch so many. He perhaps would confuse different films, or would miss details due to taking notes (if he did), or he would let his mind drift while having to sit through so many movies. But other than that, he was quite a perceptive genius and generally, his reviews were awesome. I really miss him.
That sentence of his makes no sense, really. They found a way to communicate, just not verbally, or via verbal "substitutes" (such as sign language would be). If you have a pet dog, the two of you certainly have found a way to communicate, but perhaps not in a way that would satisfy Roger Ebert. There are other ways to communicate without having to share a vocabulary of words.
However, as far as I remember, the only one to attempt to learn and use one of the other's words was the Aborigine boy who did reply that he understood the word "water" to the girl when they were at the abandoned farm (as the original poster correctly said).
An Australian school girl complete with proper school uniform back in the 1970s is highly unlikely to consider even for a moment having sex with an Aborigine boyor any boy, at that matter, but in this case, their cultures were so alien to each other that there would be no way for her to bridge that gap. And certainly his painted-up sex dance would, if anything, seal the impossibility of the deal. It just completely underscored for her that he was an uncivilized savage. Just his teenage male flesh is not enough to woo her any more than she would have been wooed by one of the kangaroos that were hopping around. So maybe what Ebert really meant to say (or imply) by his word "communicate" was that they were never able to find a common ground or bridge the gap between them. It wasn't until she was now stuck in the same boring, humdrum, materialistic life (all her husband is talking about is promotions and increased salary in some hell hole of an office), the kind of thing that probably had led to her father's suicide, that she suddenly feels a visceral yearning for all that the Aborigine boy representednot "savagery", but life in an edenic paradise.

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I liked this comment:
The cultured and the barbaric can't communicate. It's this lack of communication, an inability to converse on the same wavelength with the female, that dooms the man.

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It's implied that the father is sexually abusing the girl. It probably explains her ambivalence about his suicide.

Earlier in Ebert's review he makes this comment:
There is something subtly wrong with the family, but the film doesn't articulate it, apart from a suggestive shot of a bug that does not belong indoors.
I have no idea what he's referring to. Did I just miss that, or is he hallucinating? (Ebert has a history of getting small details wrong in his reviews).

he never replies 'Water'.
I just watched it, and yes, he does.
The problem is you all are thinking of different scenes. There's a scene where the boy gets across the point that they need water. There's as scene where he can communicate with some sign language (and in general he seems to be better at communicating with him).
But those aren't the scene the OP was referring to.

That he responded in English communicated a lot to me. I couldn't tell whether he was trying to please her, or whether he felt resigned to the fact that if he wanted to communicate with her, then it would have to be him learning English rather than her learning his language.

Fair enough.

I've just watched this, and she does ask for water, and he replies "water". It's towards the end when they are at the derelict house. She waves a can at him, and keeps repeating that he's to get water, and finally he replies with the same word.
Personally, I would hardly regard that as "communication" in any meaningful way, so I think Ebert's right.

Right, so like Roger Ebert said, the two teenagers can't even communicate via sign language - only the younger brother manages it.

No, there is a scene where the girl asks her brother how long it will take to get to somewhere, the two boys exchange some sign language and the brother replies "he says we can be there tomorrow" (or something like that).
I thought at one point there was a scene where the Aborigine boy repeats something that the girl has said, almost as if he's making a despairing effort to bridge the gap between them; or perhaps it was something the young boy had said.

If you're going to call someone an idiot for getting the details wrong, make sure you get them right yourself.
She never communicates with him, and he never replies 'Water'. As others have pointed out, the only person to use sign language is the younger brother, ie not one of the teenagers.

Roger Ebert is extremely intelligent, and next to Pauline Kael, the best movie critic ever from America.

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Well, Roger Ebert is mentally impaired, don't mind his blunders.
"I did cramps the way Meryl Streep did accents" - Calliope (Middlesex)

You are correct that in the beginning of the film, her brother communicated their needed for water. I believe the Aboriginal boy responded with the word, "Gape". However in a scene towards the end of the film, I believe they were at the farm house, she asked the Aboriginal boy for water and this time, he repeated, "Water".
"Eventually, all things merge into one, and a river runs through it." Norman Maclean

People are confusing two different scenes.

1. The younger brother is the one who makes their need for water clear, making the "glug-glug-glug" sound,
   just after they meet
   the Aboriginal boy.
2. The girl is the one who says the Aboriginal word for water, followed by the word "water", several times to the Aboriginal boy
   just before the end
   of their knowing each other. He finally repeats after her, "water, water".
   Two scenes - so, you're both right. You could say that their time of knowing the Aboriginal boy is bookended by these two brief moments of communication.

Everyone and every review says she didn't understand his dance. Apparently, we're the only two who agree she understood his dance all too well.

the girl had no way of understanding that she was basically being asked to help the Aboriginal boy make the final step into manhood and be his woman.
I'm pretty sure she understood that perfectly. Sadly for him, she wasn't up for it.
It was her younger brother who communicated their need for water, the other poster (and Ebert) was correct.
I used to want to change the world. Now I just want to leave the room with a little dignity.

As I remember it, it's the little brother who communicates their need for water to the aborigine, by pointing to his mouth and making a gurgling sound. The girl doesn't seem bright enough to do this.
Yeah, but this is a flaw in the film. You're right that she looks a bit stupid at that point, but it felt more like a heavy-handed way to make a point than an actually believable reaction. She wasn't portrayed as a terminal moron otherwise, so it didn't really make sense that she was so think-headed in that scene.
I still thought it was a good movie overall.

As I remember it, it's the little brother who communicates their need for water to the aborigine, by pointing to his mouth and making a gurgling sound. The girl doesn't seem bright enough to do this. (She just says, "Please, you must understand: we need some water.") So my feeling is that Ebert is right.

Because of the communication problem, the girl had no way of understanding that she was basically being asked to help the Aboriginal boy make the final step into manhood and be his woman. And she had no way of explaining to him, 'sorry, but we don't do it like this, and I'll be out of here as soon as we find civilization.'
She got him to understand 'water' because she wanted water. What he wanted was something that was, to a a "civilized" teenage girl, a lot more complicated.