After the sun goes nova and destroys the Earth, humans survive only in small groups on far-flung planets like Hydros, a world with no land masses at all; here, humans are permitted to share the floating islands built by the Gillies, one of several intelligent native species. But when ambitious capitalist Nid Delagard of Sorve Island accidentally kills some intelligent ``divers'' in the course of his operations, the Gillies somehow know--and order the humans off Sorve. Led by doctor Valben Lawler, in ships provided by Delagard, they sail off in pursuit of a distant uninhabited island. On the voyage, storms, tides, and deadly sea creatures take their toll of ships and people; then Lawler discovers that Delagard really intends to seek out the Face of the Waters, a fabulous, mysterious island avoided by the Gillies. At last, a handful of survivors reach the island, only to learn that the Face of the Waters is the focus of a planetary consciousness embracing all Hydran life--and they, isolated, orphaned humans, are being invited to join. This far-from-original idea raises other questions, not least the problem of a food chain whose components share a consciousness, that Silverberg declines even to acknowledge, let alone confront. So: a mediocre ocean-world odyssey--comparisons with Jack Vance's superior The Blue World are inevitable--from the veteran writer/editor, professionally turned but disappointingly superficial.