The human discovery of the earth's major land masses was mainly overland. One significant exception was crossing the waterway to New Guinea/Australia. The Pacific Islands were the very last places on earth to be settled by humans.
Travelling to the islands was impossible before advances in agriculture; as the islands were poor in resources humans needed to be able to bring and grow their own food to live there. The other crucial factor was technology; the large-scale travel required the engineering of ocean-going vessels and a system of navigation to reach the islands.
Western explorers started to explore the Pacific Ocean about 400 years ago but it was some time before they had much understanding of the Pacific. The ocean was vast, featureless, and dangerous and was viewed with fear.
Pacific peoples however, had a very different perspective; the sea provided them with abundant marine life for food and was not viewed as an obstacle, but as a highway.
Reference: Howe, KR (ed), Vaka Moana, Voyages of the Ancestors, Bateman, Auckland, 2006.