You picked myths from cultures that were geographically close so it is no surprise that the mythology looks similar. Geological evidence shows that the Black Sea was once separated from the Mediterranean, and that it appears that an earthquake ruptured the natural land dam that separated the seas, causing a massive and catastrophic flood.
Black Sea FloodAn event of this magnitude would register with all of the cultures in the area. This flood pre-dated Christianity, and would cross all religions in that area that carried the rememberance forward through the generations.
I interpreted your statement the flood is represented in a majority of religions as an opinion that the Noah type flood, with life being saved in a magical boat. This is not supported by the evidence, especially when you consider the myths from Indian tribes in North and South America.
Kwaya Flood Myth
A man and his wife had a pot which never ran out of water. They told their daugher-in-law only never to touch it, but she grew curious and touched it. It shattered, and the resulting flood drowned everything.
Total flood, no magic boat
Flood Myth of the Maya
The gods used a flood to destroy the wooden people, an early imperfect version of humanity.
No Total flood, no magic boat
Flood Myths of the Sioux
Unktehi, a water monster, fought the people and caused a great flood. The people retreated to a hill, but the water swept over them, killing them all. The blood jelled and turned to pipestone. Unktehi was also turned to stone; her bones are in the Badlands now. A giant eagle, Wanblee Galeshka, swept down, saved one girl from the flood, and made her his wife.
No Total flood, no magic boat
Flood Myth of the Yamana (Tierra del Fuego)
Lexuwakipa, the rusty brown spectacled ibis, felt offended by the people, so she let it snow so much that ice came to cover the entire earth. When it melted, it rapidly flooded all the earth except five mountaintops, on which a few people escaped. Signs of the floodwaters still show up on those mountains.
Total flood, but no magic boat