And, as Rose points out, dogs! Lots and lots of dogs! I’ve read where Cuarón said that 90% of what’s in the movie happened in real life in his childhood, even the Cleo character and her pregnancy. He shifted things around and orchestrated events to give the movie a narrative structure (of sorts), but it’s mostly drawn from personal history.
Re visual symbols: I wonder how much of it derived from his unconscious self. Obviously, some things are intentional. The airplanes, for example, I’ve read interviews where he talks about that symbolizing the great distance between Cleo (and her employee type) and the life of employers. But water? Not sure. They all build to the ocean scene with the children. Jung talks about water being symbolic of the subconscious. The fact after Cleo makes it to shore, it’s there she makes her ‘confession’: I didn’t want her to be born. Maybe there’s something more at work with water there than just the heightened drama of nearly drowning.
No matter. Your takeaway is a good one: Movies are primarily a visual medium, so always a good idea to think visually… and symbolically.