On the Feast of the Ascension in 1177, to celebrate the peace brokered by Venice between Pope Alexander III and the German emperor Frederick I Barbarossa, the pope and the doge of Venice led a procession of ceremonial barges to the Lido. The doge, symbolizing Venice, cast into the Adriatic Sea a gold ring given him by the pope. Thus, the old Venetian tradition of blessing the sea was transformed into the Sposalizio da Mare—Marriage to the Sea. Venice and the sea are indivisible; the city itself must be traversed by boat. The ritual of the Sposalizio, signifying both Venice's dependence on and dominion over the sea, is repeated annually to the present day.

Venice specialized in importing to the West products from the East, and these products arrived by sea. Its wealth depended on control of sea routes and coastal territory. Overland trade routes as well needed to be protected by skillfully negotiating trade agreements with Christian, Muslim, and Mongol powers. They also had to defend their interests against their chief maritime rivals: Constantinople, Genoa, and Pisa. The risks for Venetian merchants overseas were often great—among other things, Marco Polo encountered perilous conditions on sea and land, sickness, war, piracy, and extortion—but the rewards were also great.