Find out what? He gave up his idea at once. What could he do?

The conviction that the yacht, and everything belonging to her, were in
some indefinite but very real danger, took afresh a strong hold of him,
and the persuasion that the master of the brig was going there to
help did not by any means assuage his alarm. The fact only served to
complicate his uneasiness with a sense of mystery.

The white man who spoke as if that sea was all his own, or as if people
intruded upon his privacy by taking the liberty of getting wrecked on a
coast where he and his friends did some queer business, seemed to him an
undesirable helper. That the boat had been lowered to communicate with
the praus seen and avoided by him in the evening he had no doubt. The
thought had flashed on him at once. It had an ugly look. Yet the best
thing to do after all was to hang on and get back to the yacht and warn
them. . . . Warn them against whom? The man had been perfectly open with
him. Warn them against what? It struck him that he hadn't the slightest
conception of what would happen, of what was even likely to happen. That
strange rescuer himself was bringing the news of danger. Danger from the
natives of course. And yet he was in communication with those natives.
That was evident. That boat going off in the night. . . . Carter swore
heartily to himself. His perplexity became positive bodily pain as he
sat, wet, uncomfortable, and still, one hand on the tiller, thrown up
and down in headlong swings of his boat. And before his eyes, towering
high, the black hull of the brig also rose and fell, setting her stern
down in the sea, now and again, with a tremendous and foaming splash.
Not a sound from her reached Carter's ears. She seemed an abandoned
craft but for the outline of a man's head and body still visible in a
watchful attitude above the taffrail.