Mark Isaak has collected a large number of flood myths from across the world here. Here's one from Papua New Guinea, notice the resemblance to the Noah story:
The wife of a very good man saw a very big fish. She called her husband, but he couldn't see it until he hid behind a banana tree and peeked through its leaves. When he finally saw it, he was horribly afraid and forbade his wife, son, and two daughters to catch and eat the fish. But other people caught the fish and, heedless of the man's warning, ate it. When the good man saw that, he hastily drove a pair of all kinds of animals into trees and climbed into a coconut tree with his family. As soon as the wicked men ate the fish, water violently burst from the ground and drowned everyone on it. As soon as the water reached the treetops, it sank rapidly, and the good man and his family came down and laid out new plantations.
Why is the flood myth so common in folklore? Why the focus on floods rather than, say, earthquakes, landslides, or forest fires? Possibly because most early civilizations were close to sources of water – seas, rivers, lakes – and so more likely to be affected by floods than other natural disasters.
Then the harder question is why the structure found in the Noah story is so common: a deity causes the flood because of mankind's wickedness, a small number of good people are allowed to survive, the survivors then repopulate the Earth. What makes this such a successful structure for folklore?