This sounds very romantic, but they were mostly at drift. Most very using rafts without any form of propulsion. "Navigation" across wast distances was one way road with no return ticket. They had to do it for overpopulation, not for some explorative spirit.
There is plenty of evidence that Pacific Islanders were able to navigate from island to island, for example in the book I cited above. Here's more [1]. What's your evidence for them not being able to?
Honestly, this sounds like some crap spread by people who can't bring themselves to believe that non-Western "primitive people" could have highly developed skills.
I don't have references easily to hand but I don't think that's true. Polynesian navigators were able to make repeat journeys to the same destination reliably enough to return home and build maps and train successors in their craft.
The Polynesians used double-hulled canoes not rafts. King Kamehameha conquered the Hawaiian island using giant canoes with accounts from Westerners.
The system of navigation was recorded orally. People in living memory retained the knowledge. Anthropologists got it from them, built replica canoes, and then sailed around discovering that it worked. Captain Cook used Polynesian navigator who was well traveled.
Maybe the rafts are coming from Easter Island losing the technology and using rafts instead.
