Were Pacific Islander cultures ever sufficiently remote/detached to consider the entire world as simply an ocean dotted by chains of islands? If so, did they have a name or ideological/religious concept for this theoretically "endless ocean"?

There is archaeological evidence that the Polynesian peoples who settled the various Pacific islands did remain in contact with each other, despite some divergence in languages and culture over the centuries of individual settlements. The first thing it's important to remember is that these people were seriously skilled navigators, who knew how to use the stars, currents, winds, the flight patterns of birds, etc. to get from place to place. They weren't just randomly sending out canoes scatter-fashioned in the hope of hitting something. Settlement expeditions were planned after initial discoveries were made by people sent out to explore along known routes. We know this because necessary supplies were taken along from the home site~ seeds, breeding pairs of domesticated animals, etc.

We have evidence that places "kept in touch" from several sources. Oral tradition, indicating that contact was maintained, at least to some degree, between Tahiti and the Hawaiian Islands. There is evidence in the oral traditions of Hawaii that family visits took place, remains were returned to the home islands for burial, sacred soil and stones were moved between the two places, etc. Archaeological evidence~ stone tools have been found in French Polynesia that originated in Hawaii and on distant Pitcairn Island, evidenced by the fact that the type of stone used is only found in those places. Petroglyph art that has historical origins in the Hawaiian Islands has been identified in Tonga and Tahiti. Evidence of varying quality (linguistic, archaeological, genetic) has even been introduced that there was at least one-time contact between the people living on Rapa Nui (Easter Island) and the western coast of South America. This last point should be taken with caution as of now, as the evidence remains circumstantial, and no definitive genetic evidence has emerged.

I don't mean to suggest here that there was a steady flow of vessels traveling constantly between these island chains, but contact was maintained to one degree or another for centuries after colonization. So to your original question~ I don't know that any of the places Polynesian peoples settled were isolated long enough for all memory of "other places" to fade, and the archaeological and historical record is indicative of sustained, though at times sporadic, contact.

Sources:

Kirch, Patrick Vinton. On the Road of the Winds. Berkeley: University of California Press, 2002. This is a great source in general about the settlement of the Pacific Islands, and provides lots of information on archaeology in particular (or, at least, the state of things when it was published in 2002).

This excellent collection of oral histories at the University of Hawaii's website, from the out of print Voyaging Chiefs of Havai‘i (1995). Someone kindly digitized it. Loads of oral tradition/travelogues collected in the 19th and 20th centuries.

Here's a peer-reviewed article from Nature about the stone tools originating in Hawaii being found in Tahiti.

Note: I'm a biology professor with an interest in this stuff, not a historian or archaeologist, so I'm happy to hear any criticisms people have on this.