Extra question: Was there contact between China and 'Polynesian' cultures in earlier time periods? Specifically pre-16th century? Zhang He's voyages seem to consume so much of Chinese maritime history, but they seem most notable because they went West. Surely someone went East?
For this question, I'm using the Polynesian Triangle definition from Wiki (Hawaii, New Zealand, and Easter Island).
I don't think that your question about what Western explorers thought about it has been answered, but there's several answers by /u/b1uepenguin about Pacific Islanders in the FAQ, including one on 'Did the Polynesians trade with East Asian civilisations like the Chinese or Japanese?' and a few on what their navigation techniques actually were, e.g., (this one on 'How could Polynesians navigate so well without compasses or sextants and how could they build ships capable of traveling deep ocean?'. Hope that helps!
As a follow-on question, is there a definitive resource for understanding Polynesian navigational techniques?