Although we can't emphasize enough how journalistic and fact-oriented Hersey's narrative is, he manages to find and highlight some impressive symbolism in the pure hard facts of The Bomb's aftermath.
For example, he shows how completely upside-down the post-bombing world was by emphasizing the fact that two things you would normally associate with life and awesomeness—that is, light and water—became sinister and even toxic in the bomb's aftermath.
First, let's check out what he does with water. In his description of Mrs. Nakamura's reaction to the blast, Hersey draws attention to what water symbolized pre-blast for residents of Hiroshima. When Mrs. Nakamura realized she could not bring her most prized possession, her sewing machine, with her to evacuate, she:
… plunged her symbol of livelihood into the receptacle which for weeks had been her symbol of safety—the cement tank of water in front of her house, of the type every household had been ordered to construct against a possible fire raid. (2.8)
Sure, it's kind of baffling that Mrs. Nakamura would throw a metal sewing machine into a water tank, but as Hersey says, she (and probably many others, too) associated the tank/the water in it with life and safety. And we probably wouldn't be thinking too clearly about rust if you had just survived an atomic attack, either, so no judgement.
In the post-bomb Hiroshima, though, Hersey suggests that water was often more dangerous than helpful. For example, people evacuated near/in the river (to seek refuge from the fires everywhere) ran the risk of drowning. Case in point: Mr. Tanimoto had to help move some people (who were too frail to move themselves) off a sandspit, where the tide was advancing toward them.
Unfortunately, the next morning, he discovered that they had drowned nonetheless:
The tide had risen above where he had put them; they had not had the strength to move; they must have drowned. He saw a number of bodies floating in the river. (3.15)
So, yeah, while water was certainly a refuge for people trying to escape/put out surrounding fires, it was incredibly dangerous for those who either couldn't or were too weak to swim in it. It was definitely not totally a symbol of life anymore.
Then, of course, we have the moment when Mrs. Nakamura and her children drank from the river and were super disgustingly sick afterwards:
They all felt terribly thirsty, and they drank from the river. At once they were nauseated and began vomiting, and they retched the whole day. (2.46)
Okay, it's not really clear that the water itself caused their actual illness, but the fact that this normally life-giving thing caused them to vomit definitely contributes to our general sense that water's meaning—and life in general—has gone all funky for Hiroshima after the bombing.
Something similar happens with the story's references to light. Although you probably associate light with good things like life (à la "Let there be light!") and sunshine, it's actually pretty sinister in Hiroshima, since the bright flash from the explosion had all kinds of damaging and weird effects and after effects.
The residents of Hiroshima soon realized that there was something funky about that flash—it definitely wasn't your garden-variety burst of light. Doctors in the Red Cross hospital were clued into the special nature of the bomb's light when they started exploring the hospital after the explosion:
Word went around among the staff that there must have been something peculiar about the great bomb, because on the second day the vice-chief of the hospital went down in the basement to the vault where the X-ray plates were stored and found the whole stock exposed as they lay. (3.39)
Although the flash certainly wasn't visible from within the basement, its effects (i.e., the radiation) managed to penetrate within. So, you get the picture—this bomb and its "light" was anything but typical.
Also, the flash left weird permanent shadowy marks on buildings all around the city:
The experts found… a permanent shadow thrown on the roof of the Chamber of Commerce Building (220 yards from the rough center) by the structure's rectangular tower; several others in the lookout post on top of the Hypothec Bank (2,050 yards); another in the tower of the Chugoku Electric Supply Building (800 yards); another projected by the handle of a gas pump (2,630 yards); and several on granite tombstones in the Gokoku Shrine (385 yards). (4.10)
Hersey keeps things pretty journalistic in the book, but this reference to "permanent shadows" cast all around the city seems to have both literal and symbolic meaning; in addition to leaving actual marks on the buildings, the bomb light has cast metaphorical "permanent shadows" on the city's people.
As with his references to water, the way Hersey takes something that's supposed to be awesome and healthy like light and makes it toxic and sinister kind of highlights the way the whole world post-atom bomb appears to have gone all sorts of topsy-turvy.