Witch of the Demon Seas by Poul Anderson

Under a hot sullen sky, the windless sea swelled in long slow waves that rocked the tangled kelp and ocean-grass up and down, heavenward and hellward. To starboard, the dark cliffs of a small jungled island rose from an angry muttering surf, but there were no birds flying above it.

Corun pointed to the shore. "That's the first of the archipelago," he said. "From here on, we can look for the Xanthi to come at any time."