[jamiek88]

Imagine how different naval history would be if the Megalodon still existed and considered humans / boats prey.

The size of vessel needed to be safe would be mind boggling. Would we have had to wait for steel to be invented?!

[semi-extrinsic]

Implausible that it would have changed much. People went hunting for sperm whales in small wooden boats. Sperm whales are even bigger than Megalodon. Sure they're peaceful ordinarily, but when you start throwing harpoons into them, not so much.