Mars is dry. We see this right away when Ylla is riding into town: she "didn't watch the dead, ancient bone-chess cities slide under, or the old canals filled with emptiness and dreams. Past dry rivers and dry lakes they flew" ("Ylla," 76). So, dryness is associated here with an old, ancient civilization, something dried up and used out.
And that's actually the way Mars was imagined in the early 1900s. (See "Setting" for more about that.) Some astronomers thought that Mars was full of dried-up channels that used to ferry water to ancient cities.
So, in a way, all this dryness seems to represent some possible future vision of Earth. If Mars used to be like earth—the way Spender thinks that "one day Earth will be as Mars is today" ("—And the Moon Be Still as Bright," 81)—then Mars's dryness represents a future.
That becomes explicit in "Night Meeting," when Tomás meets a Martian who's horrified to hear what happened to Mars: "A man can face the Past," Muhe Ca says, "but to think—the pillars crumbled, you say? And the sea empty, and the canals dry, and the maidens dead, and the flowers withered?" ("Night Meeting," 125).
(Yeah, obviously the most important thing here is that the maidens are dead. Yikes, Bradbury. Maybe tone down the totally obvious sexism?)
Anyway, the point is that water = life, the future, hope, youth, all the good things in life; and dry = death, the past, despair, age, all the bad things. That's certainly how it's used in "The Green Morning," when Driscoll's languishing seeds spring up after one good rain.
But what about "The Martian"? It rains at the beginning of the story, when the LaFarges find their "son" (rain=good). But it also rains at the end of the story, when they lose him again. And in "The Million-Year Picnic," the dad shows his family that "the Martians were there—in the canal—reflected in the water. Timothy and Michael and Robert and Mom and Dad" (138). But we already know that's a kind of ambiguous ending.
So, like fire, water seems to stand for both good and bad things—both life and loss. Or maybe water just does what it does, ignoring people—as in the title poem of "There Will Come Soft Rains." We like this interpretation, because it makes water the opposite of fire. Where fire always seems to be controlled by people, either for good or bad purposes, water does its own thing. It's truly a force of nature.