A deluge that destroys humanity appears in hundreds of cultures, from Mesopotamia to the Maya to China.
Documented
Shared structure, local disasters
Over 200 flood traditions are documented worldwide. Geology knows of catastrophic regional floods: Mesopotamian river floods, the Black Sea deluge hypothesis around 5600 BC, and the colossal meltwater floods at the end of the Ice Age that raised seas 120 meters and drowned countless coastlines where early humans lived.
Assessment: No evidence for one simultaneous global flood. Strong evidence that real catastrophic floods, and the drowned Ice Age coastlines, seeded global memory.
Stories
One event, many memories
Many traditions insist the stories descend from a single historical deluge: Noah, Utnapishtim, Deucalion, Manu. The Epic of Gilgamesh's flood predates Genesis and closely parallels it.
Assessment: The literary link between Gilgamesh and Genesis is well established. A single worldwide event is not supported by geology.
Interesting claims
A reset of a prior world
Fringe theories read the flood as the destruction of an advanced antediluvian civilization, sometimes by comet impact at the Younger Dryas 12,900 years ago.
Assessment: The Younger Dryas climate event is real science; the impact cause is contested; the advanced civilization is unevidenced.
Sources and further reading
Epic of Gilgamesh, tablet XI, Utnapishtim's account
Genesis 6-9
Atrahasis tablet III
Worth asking
What would it mean for human memory if the oldest story we all share is a disaster?