RevDG writes:
most people who live near the sea have more water based myths, because they live near water and they live by fishing
I think that is an important point when considering the universal flood myths, as well as when considering why they are so, well, universal. Also, most human settlements are coastal, along a river, or, like New Orleans, both. In the U.S., 53% of the population lives in a coastal county; the percentage that lives within a short drive of the coast is even higher. No other natural catastrophe has ever been more likely in human history than a flood.
Further, one must consider the relatively smaller "size" of the ancient world. This was a world without any conception of the earth as a sphere. There was the land. There was the sky. There was the water.
For example (chosen since I have known these flood waters), the flood plain of the region about the conflux of the Mississippi and Ohio rivers is 81 miles across. According to the superduper formula for distance to the horizon:
1.17 times the square root of your height of eye
=
distance to the horizon in nautical miles
A person clinging to a tree on that fully flooded plain, say 9 feet above the water, would see nothing but water, water everywhere, since the distance to the horizon would be approximately 3.5 nautical miles, or approximately 4 landlubber miles. Even a 100' perch would only let him see 11 nautical miles across the vast flood plain.
So our survivor could float across the flood plain of a great river for many hours or days and never see anything but water in all directions. I don't know what the tribal mind would consider universal, but I suspect that would do. Also, since the feeding tributaries in great floods also themselves flood, the reports from afar would concur that the flood was everywhere.
The universality of the flood myth springs from the universality of floods in the areas where humans choose to settle. Our size, and the size of those floods, would make the floods--especially the great 1000 year or 10,000 year floods--seem to have swallowed the world entire. Only the gods could explain events of this scale.