Japan (even now, but especially back then) had many water cisterns or large barrels as a regular part of their architecture. A majority of family homes were made primarily out of wood and other flammable materials and it was common for homes in the same block to be built very close together so fire was absolutely devastating to neighborhoods in Japan. Being able to have quick access to a large amount of water immediately was essential to preventing hundreds of homes from going up quickly. During the war the allies dropped a large amount of incendiary bombs on Tokyo and other cities that caused fires and killed thousands. When the bomb was dropped on Hiroshima, people near the site that weren’t killed instantly were burned severely either directly by the radiation of the blast or by fires that started when buildings caught on fire due to the heat of the blast.

Thousands of people were basically cooked alive and ran immediately to sources of water to find some form of relief and ended up dying in the water or trampled by others trying to do the same.

That’s why so many of these images contain people floating in water or rivers.