What is actually going on here in this passage? What is the context in the novel? What is the myth, and how does that serve as a kind of allegory or metaphor for what is being described here? "Heyst had never been so much astonished in his life. He stared dumbly at the strange boat's crew. . . . The civilization of the tropics could have had nothing to do with it. It was more like those myths, current in Polynesia, of amazing strangers, who arrive at an island, gods or demons, bringing good or evil to the innocence of the inhabitants—gifts of unknown things, words never heard before."

This passage describes the moment when Heyst sees the "white man boat." He was alerted to its presence by Wang, his Chinese servant. Heyst is so alarmed by the arrival of the boat that it seems to him a "strange hallucination," a "phantom boat." At this point in the novel, Heyst has been living on his island for some time with Wang and Lena. He is absolutely not expecting the arrival of the boat, which he finds extremely "unlikely." When Heyst goes to investigate, he finds three men in the boat who are "not sailors." In this passage, Heyst expresses his inability to connect their appearance "with anything plausible." These men seem very out of place to him: he describes them as "like those myths . . . of amazing strangers, who arrive at an island . . . bringing good or evil."
The myth alluded to is one common to Polynesia and other islands: when strangers arrive from other places, they may bring good or evil, part of the myth cycle which sets the native in opposition to the stranger. In the Polynesian language, "Tahiti" actually means "strange land," and in Polynesian mythology, the first stranger came to their islands from a distant place and either overthrew the ruling chief there by fighting him or married his wife to usurp him. It is not possible, in these myths, to know whether a stranger is bringing good or evil until they have revealed themselves. In the context of this novel, then, Heyst feels a similar uncertainty about who these strangers are and what they represent—whether they will bring him "good or evil." In this case, the three men turn out to be harbingers of evil. Ricardo, Jones, and Pedro will ultimately lead to the death of the one thing that brings meaning to Heyst's life, his lover, Lena. At the end of the novel, Lena dies as a result of a stray bullet fired by Jones, and thus the death of Heyst's lover as a result of the arrival of strangers mimics, to a certain extent, the Polynesian myth of strangers coming to an island and destroying paradise.
See Andersen's Myths and Legends of the Polynesians for more on the mythology referenced.