In the coming months, top river-cruise lines will begin charting a course previously traveled only by oceangoing vessels. AmaWaterways and Uniworld will both launch globe-spanning world cruises that take their peripatetic passengers on voyages best measured not in days but weeks, not in miles but leagues. Each will bring sophisticated travelers on journeys through the planet’s most captivating rivers and canals, making daily stops for intimate shore-side exploration of the villages, cities and countryside along the way.