I'm constructing a world where the humanity had to migrate from land to water somewhere in antiquity. In this world, humans are living on seas and rivers. They live both on pier-like and other fixed platforms near the coast as well as on rafts strung together into larger settlements offshore. Fishing and sea-plant farming (algae etc.) are the main sources of food. Ships and boats are pretty much the only means of transportation.
The land still exists (topography is the same as of the real world) - no "Water World" scenario. However, humans only visit it for brief periods (several hours, maybe up to half a day) and only to hunt/gather/acquire wood and other land-only resources. There are no permanent human settlements on land. Obviously, this makes certain activites tricky (land farming, mining, heavy industry) but the technical consequences will be deal with in a separate question.
Other life on land exists more or less unchanged (some modifications are allowed to enable a viable answer but I don't want a mass extinction of land-based life).
My question is: What event (can be sudden or gradual) could cause such a migration happen?
One caveat is that the event must be global (or eventually global) so that humans all over the world are affected and move their lives to water independently and without communication.
Several thoughts of my own (ideas and associated problems):
- Predators: Humans in real world are apex predators primarily due to the highly developed intelligence and social structure. If a smarter and more dangerous predator has evolved on land, why hasn't it taken to the sea to pursue its prey?
- Superstitions: No actual monsters on land but tales of such passed from generations are preventing people from returning. If so, how did these originate in the first place? And, given human nature, what would prevent people to go against the "wisdom of the fathers", settle on land and eventually bust the myth?