Yet another commercial sequel-of-a-sequel--with the old Herbert ideas wearing rather thin now. Five-hundred years after Ship (Destination: Void, 1966) abandoned planet Pandora, the land masses have washed into the sea and the clone-descended population is divided into: the genetically variable Islanders, bio-engineering wizards who live on crowded, floating organic islands; and the submarine, high-tech, human-normal Mermen. But the latter are secretly restoring the kelp which once controlled the seas, using it to cement the new continents they're building--so the randomly-drifting islands are doomed. GeLaar Gallow, a venal Merman, favors wiping out the despised, mutant Islanders by sinking their islands; he's also planning an expedition to broach the ""hyb"" tanks left in orbit by Ship, where he hopes to find plenty of willing, normal recruits preserved in suspended animation. Wise old Islander judge Ward Keel and sympathetic Merman Shadow Panille ineffectively oppose Gallow (an island is disastrously sunk). . . but then the kelp reaches critical mass and once again becomes sentient. And it's directed by the human/kelp descendant Vats, kept alive in a tank on an island since The Jesus Incident (1979); the kelp absorbs the personalities of the dead, thus--per the title--offering immortality. Intelligent, as almost always, with solid characters and carefully constructed scenarios--and, though uneven, likely to follow the other Herberts to best-sellerdom, especially since it's more straightforward and accessible than his usual.