Although I can't prove it....
...I have reason to believe they knew the world was round, and had a rough idea about how big it was. It was probably the last secret the master wayfarers passed on to their apprentices when they felt death was near. They knew the universe was not just a giant lake dotted with islands out to infinity. Although they had probably never been near enough to a continent to see it, they knew a vast abyss of something bordered the eastern Pacific. They could sense it in the waves and currents, the weather, and in the movements of sea life, and they feared it.
On those long night watches at the helm, under the pitiless stars, you have a lot of time to think.