The question really relates to why the Indian Ocean and such waters was travelled before the Atlantic and the Pacific, as no ocean is actually impassable. The answer lies in the technology being used to navigate the waters. In early marine travel ships would stay close to land for the following reasons 1) Poor ability to sail into the wind, and the risk of becoming becalmed. Thus there existed the real possibility of becoming marooned in the middle of an ocean unable to return to land. This fear was particularly heightened by the poor understanding of how winds were generated (usually attributed to gods) 2) The need to seek shelter in storms. Early boats were not capable of withstanding severe storms and required to be sheltered in severe weather. 3) Limited navigation - limited ability to tell direction, no ability to tell longitude. Travelling in unknown waters far from land meant having no idea where you were, and almost no idea how to get back.
The Indian Ocean is thus easy to navigate by staying close to the shoreline from Africa to Indonesia, and thus this is how it was done. Australia was visited only accidentally through being lost.
An Atlantic crossing requires a leap of faith in very risky waters. 308
<PERSON> achieved it through luck, and a combination of delusion and information gathered from the fishing community. It is possible the Portuguese fishermen where regularly travelling to the West Atlantic. This knowledge being built up over centuries of experience and the quest for wealth. The Vikings achieved it by travelling in small steps across the far north. This involved expertise in travelling through such waters that only the Vikings possessed, and reliance on rowing as well as sail.
The North Pacific was not crossed as there was no corresponding culture to the Vikings capable of cold sea navigation. The central Pacific was navigated, however, by the Polynesians. This was done gradually over centuries, driven by population pressures, by people expert in navigating such areas, capable of living off the sea, and by using rowing as the primary means of locomotion. It was done in a step-wise manner, from island to island looking for tell-tale cloud formations as navigation. There was likely significant loss of life in such travels.
So the navigability of the world's oceans is an expression of the culture and technology of the people travelling, with the short answer that it is relative easy to travel along the coast in warm climates. 45