It was much cooler I think in those days and the seas were much calmer than now. That would explain crossng an ocean in a catamaran
Polynesians may not have had a lot of technology but they knew the sea. I’ve seen photos of maps they made out of reeds and grass that show detailed wave and wind patterns as well as islands.
As I understand it, people with expert knowledge of the sea can tell if they’re approaching an unseen island by the behavior of swells.