the Polynesians were the greatest navigators ever, far better than the Vikings (who were seldom more than a day or two out of sight of land). They went in three great waves over about 1000 years, as I recall: one from somewhere round Taiwan to somewhere in the mid-Pacific, then on again to Hawaii, then back south-west to New Zealand. En route they almost certainly did get to South America, becasue they brought back sweet potatoes. They would have followed the stars, the currents and the winds; they might have spotted an island several days off because of birds flying above it.
I've flown over the Pacific and it's vast. Even at 500mph or so, you can go for hours with absolutely nowhere showing up on the in-flight maps. The idea of sailing on it in outriggers, and not knowing what you might find, is awe-inspiring.