Despite the series being set on an unnamed Pacific island, it's clear from the vegetation and the terrain shown that it was in fact on the mainland of California. This is most obvious during fantasies which take place mostly in outdoor settings.
The "lagoon" where the seaplane arrives bringing each week's guests has no tide, nor wave action. In fact, it is so placid and calm that it is clearly a large pond on a controlled set and not the waterway on or near an island.
Despite the setting for the series being a tropical island, there are many episodes where the characters wear clothing that would be very uncomfortable in a tropical climate.