Centuries ago, in an effort to flee barbarians, Venetians left dry-dock and drifted out to a flotilla of "uninhabitable" islands in the lagoon. Survival was difficult enough, but no Venetian has ever settled for mere survival. The remote ancestors of the present inhabitants created the world's most beautiful city. To your children's children, however, Venice might be nothing more than a legend. The city is sinking at an alarming rate of about 2 1/2 inches per decade, and at the same time, the damp climate, mold, and pollution here are contributing to the city's decay. Estimates are that, if no action is taken soon, one-third of the city's art will deteriorate hopelessly within the next decade or so.