The Merchant of Venice by William Shakespeare is set in Renaissance Venice, which was one of the great centers of mercantile life in Europe. Venice was a strong sea power. As a result of the Fourth Crusade, Venice dominated much of the former Byzantine holdings, and controlled the sea routes through the Bosphorus as well as being active in trading the the Adriatic and Aegean seas.
Shakespeare's portrait of Venetian mercantile life reflects many aspects of the realities of the period. Due to Roman Catholic prohibitions on usury, money lending was a primarily Jewish occupation, as we see exemplified by Shylock.
International and long distance trade routes were the prerogatives of the nobility. Trade could bring vast wealth, but at great risk, as ships were subject to the vagaries of weather and the omnipresent dangers of piracy. Thus the portrait of Antonio as having unpredictable fluctuations in his vast wealth and needing recourse to money lenders is quite accurate.