Venice came to life on spongy mudflats at the edge of the habitable world. Protected in a tidal estuary from barbarian invaders and Byzantine overlords, the fishermen, salt gatherers, and traders who settled there crafted an amphibious way of life unlike anything the Roman Empire had ever known. In an astonishing feat of narrative history, James H. S. McGregor recreates this world-turned-upside-down, with its waterways rather than roads, its boats tethered alongside dwellings, and its livelihood harvested from the sea.

McGregor begins with the river currents that poured into the...